Firefox/Tweaks

Related articles

• Firefox
• Browser plugins
• Firefox/Profile on RAM
• Firefox/Privacy

This page contains advanced Firefox configuration options and performance tweaks.

Performance

Improving Firefox's performance is divided into parameters that can be inputted while running Firefox or otherwise modifying its configuration as intended by the developers, and advanced procedures that involve foreign programs or scripts.

Note: Listed options may only be available for the latest version of Firefox.

This section contains advanced Firefox options for performance tweaking. For additional information see these MozillaZine articles.

Change Performance settings

Firefox automatically uses settings based on the computer's hardware specifications [1].

Adjusting these settings can be done in Preferences or by changing the dom.ipc.processCount value to 1-8 and browser.preferences.defaultPerformanceSettings.enabled to false manually in about:config.

However you may want to manually adjust this setting to increase performance even further or decrease memory usage on low-end devices.

In this case the Content process limit for the current user has been increased to 4:

$ ps -e | grep 'Web Content'

13991 tty1     00:00:04 Web Content
14027 tty1     00:00:09 Web Content
14031 tty1     00:00:20 Web Content
14040 tty1     00:00:26 Web Content

Enable OpenGL Off-Main-Thread Compositing (OMTC)

Basic software OMTC is enabled by default.

To check if OpenGL OMTC is enabled, go to about:support and under the "Graphics" section look for "Compositing". If it reports "Basic", OpenGL OMTC is disabled; if it reports "OpenGL" it is enabled.

If OpenGL OMTC is disabled, you can force-enable it by going to about:config and enabling layers.acceleration.force-enabled. Restart Firefox for changes to take effect.

Note: If OpenGL OMTC is disabled for a specific hardware, it may be due to stability issues, high system resources consumption, driver bugs or a number of different variables, and so instead of speeding things up it might slow them down. Proceed with force-enabling it at your own risk, benchmark if you aren’t sure.

For more information on OMTC in Firefox read MozillaWiki:Platform/GFX/OffMainThreadCompositing.

Enable WebRender

Enables WebRender compositor in Servo, instead of Gecko. It improves performance on supported hardware, fixes some black visual glitches when "OpenGL Off-Main-Thread" is enabled. Currently it has some known bugs. You can read more in MozillaWiki:Platform/GFX/Quantum Render.

Mozilla is slowly deploying WebRender to "qualified" Linux configurations. The qualification mostly depends on the GPU, the driver's version and the screen resolution.

One can force-enable it using one of the two methods below:

1. by setting the MOZ_WEBRENDER environment variable to 1 before launching Firefox.
2. by going to about:config and setting gfx.webrender.all to true. Restart Firefox for changes to take effect.

To check if WebRender is enabled go to about:support and under the "Graphics" section, in the "Decision Log" sub-section, look for "WEBRENDER". If it reports "available by user: Force enabled by pref", it means is enabled. Otherwise it will just report "opt-in by default: WebRender is an opt-in feature".

Note: If WebRender is disabled for a specific hardware, it may be due to stability issues, high system resources consumption, driver bugs or a number of different variables, and so instead of speeding things up it might slow them down. Proceed with force-enabling it at your own risk, benchmark if you are not sure.

Enable Accelerated Azure Canvas

Warning: Accelerated Azure Canvas may cause invalid/corrupt rendering of images on unsupported devices and/or drivers, see #Enable OpenGL Off-Main-Thread Compositing (OMTC).

Go to about:config, accept the warning, right click and create a new boolean value. Set the name as gfx.canvas.azure.accelerated and set it to true.

To verify restart Firefox then go to about:support and search for AzureCanvasAccelerated which should be set to 1.

The acceleration efficiency can be tested by comparing the speed of a javascript demo with and without the setting.

Stop urlclassifier3.sqlite from being created again

Removing all urlclassifier* files can prevent the use of megabytes of storage in your firefox profile. If you remove all the urlclassifier* files, you may find out that urlclassifier3.sqlite keeps growing again after a certain time. Here is a simple solution to avoid it for now and ever.

$ cd ~/.mozilla/firefox/<profile_dir>
$ echo "" > urlclassifier3.sqlite
$ chmod 400 urlclassifier3.sqlite

This effectively makes the file empty and then read-only so Firefox cannot write to it anymore.

Turn off the disk cache

Every object loaded (html page, jpeg image, css stylesheet, gif banner) is saved in the Firefox cache for future use without the need to download it again. It is estimated that only a fraction of these objects will be reused, usually about 30%. This is because of very short object expiration time, updates or simply user behavior (loading new pages instead of returning to the ones already visited). The Firefox cache is divided into memory and disk cache and the latter results in frequent disk writes: newly loaded objects are written to memory and older objects are removed.

The disk cache can be turned off by enabling Preferences > Privacy & Security > Cached Web Content - Override automatic cache management and setting to 0 Limit cache to ... MB of space.

An alternative approach is to use about:config settings:

• Set browser.cache.disk.enable to false
• Verify that browser.cache.memory.enable is set to true, more information about this option can be found in the browser.cache.memory Mozilla article
• Add the entry (right click > new > integer) browser.cache.memory.capacity and set it to the amount of KB you want to spare, or to -1 for automatic cache size selection (skipping this step has the same effect as setting the value to -1)
  • The "automatic" size selection is based on a decade-old table that only contains settings for systems at or below 8GB of system memory. The following formula very closely approximates this table, and can be used to set the Firefox cache more dynamically: 41297 - (41606 / (1 + ((RAM / 1.16) ^ 0.75))), where RAM is in GB and the result is in KB.

This method has some drawbacks:

• The content of currently browsed webpages is lost if the browser crashes or after a reboot, this can be avoided using anything-sync-daemon or any similar periodically-syncing script so that cache gets copied over to the drive on a regular basis
• The settings need to be configured for each user individually

Longer interval between session information record

Firefox stores the current session status (opened urls, cookies, history and form data) to the disk on a regular basis. It is used to recover a previous session in case of crash. The default setting is to save the session every 15 seconds, resulting in frequent disk access.

To increase the save interval to 10 minutes (600000 milliseconds) for example, change in about:config the setting of browser.sessionstore.interval to 600000

To disable completely this feature, change browser.sessionstore.resume_from_crash to false

Referrer header control

The HTTP Referer header can be extensively configured via about:config. See MozillaWiki:Security/Referrer for the available preferences.

Defragment the profile's SQLite databases

Warning: This procedure may damage the databases in such a way that sessions are not saved properly.

Firefox keeps bookmarks, history, passwords in SQLite databases. SQLite databases become fragmented over time and empty spaces appear all around. But, since there are no managing processes checking and optimizing the database, these factors eventually result in a performance hit. A good way to improve start-up and some other bookmarks and history related tasks is to defragment and trim unused space from these databases.

You can use profile-cleaner^AUR to do this, while Firefox is not running:

profile-cleaner usage example:

SQLite database	Size Before	Size After	% change
urlclassifier3.sqlite	37 M	30 M	19 %
places.sqlite	16 M	2.4 M	85 %

Firefox provides a tool to defragment and optimize the places database, which is the source of most slowdowns and profile corruptions. To access this tool, open the about:support page, search for Places Database and click the Verify Integrity button.

Cache the entire profile into RAM via tmpfs

If the system has memory to spare, tmpfs can be used to cache the entire profile directory, which might result in increased Firefox responsiveness.

Disable Pocket

If you do not use the Pocket-service, you may want to disable it by setting extensions.pocket.enabled to false in about:config.

Appearance

Fonts

See the main article: Font configuration

Configure the DPI value

Modifying the following value can help improve the way fonts looks in Firefox if the system's DPI is below 96. Firefox, by default, uses 96 and only uses the system's DPI if it is a higher value. To force the system's DPI regardless of its value, type about:config into the address bar and set layout.css.dpi to 0.

Note that the above method only affects the Firefox user interface's DPI settings. Web page contents still use a DPI value of 96, which may look ugly or, in the case of high-resolution displays, may be rendered too small to read. A solution is to change layout.css.devPixelsPerPx to system's DPI divided by 96. For example, if your system's DPI is 144, then the value to add is 144/96 = 1.5. Changing layout.css.devPixelsPerPx to 1.5 makes web page contents use a DPI of 144, which looks much better.

See also HiDPI#Firefox for information about HiDPI displays and [2] for calculating the DPI.

Default font settings from Microsoft Windows

Below are the default font preferences when Firefox is installed in Microsoft Windows. Many web sites use the Microsoft fonts.

Proportional: Serif Size (pixels): 16
Serif: Times New Roman
Sans-serif: Arial
Monospace: Courier New Size (pixels): 13

General user interface CSS settings

Firefox's user interface can be modified by editing the userChrome.css and userContent.css files in ~/.mozilla/firefox/<profile_dir>/chrome/ (profile_dir is of the form hash.name, where the hash is an 8 character, seemingly random string and the profile name is usually default).

Note:

• The chrome/ folder and userChrome.css/userContent.css files may not necessarily exist, so they may need to be created.
• toolkit.legacyUserProfileCustomizations.stylesheets must be enabled in about:config.

This section only deals with the userChrome.css file which modifies Firefox's user interface, and not web pages.

Change the interface font

The setting effectively overrides the global GTK font preferences, and does not affect webpages, only the user interface itself:

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

* {
    font-family: "FONT_NAME";
}

Hide button icons

Enables text-only buttons:

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

.button-box .button-icon {
    display: none;
}

Hiding various tab buttons

These settings hide the arrows that appear to the horizontal edges of the tab bar, the button that toggles the "all tabs" drop-down list, and the plus sign button that creates a new tab.

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

/* Tab bar */

.tabbrowser-strip *[class^="scrollbutton"] {
    /* Hide tab scroll buttons */
    display: none;
}

.tabbrowser-strip *[class^="tabs-alltabs"] {
    /* Hide tab drop-down list */
    display: none;
}

.tabbrowser-strip *[class^="tabs-newtab-button"] {
    /* Hide new-tab button */
    display: none;
}

Horizontal tabs

To place the tab bar horizontally stacked along the sides of the browser window:

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

/* Display the tabbar on the left */
#content > tabbox {
    -moz-box-orient: horizontal;
}

.tabbrowser-strip {
    -moz-box-orient: vertical;
    /*
     * You can set this to -moz-scrollbars-vertical instead,
     * but then the scrollbar will *always* be visible.  this way
     * there is never a scrollbar, so it behaves like the tab bar
     * normally does
     */
     overflow: -moz-scrollbars-none;
}

.tabbrowser-tabs {
    -moz-box-orient: horizontal;
    min-width: 20ex;   /* You may want to increase this value */
    -mox-box-pack: start;
    -moz-box-align: start;
}

.tabbrowser-tabs > hbox {
    -moz-box-orient: vertical;
    -moz-box-align: stretch;
    -moz-box-pack: start;
}

.tabbrowser-tabs > hbox > tab {
    -moz-box-align: start;
    -moz-box-orient: horizontal;
}

Hide title bar and window border

To replace the title bar with the tab bar, enable browser.tabs.drawInTitlebar in about:config.

Or go to "Menu", then "Customize" and then at the bottom-left corner find checkbox named "Title Bar". Uncheck it. If the checkbox is missing, make sure the XDG_CURRENT_DESKTOP environment variable is correctly set and/or the MOZ_GTK_TITLEBAR_DECORATION environment variable is set to "client".

Auto-hide Bookmarks Toolbar

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

#PersonalToolbar {
    visibility: collapse !important;
}

#navigator-toolbox:hover > #PersonalToolbar {
    visibility: visible !important;
}

Remove sidebar width restrictions

~/.mozilla/firefox/<profile_dir>/chrome/userChrome.css

/* remove maximum/minimum  width restriction of sidebar */
#sidebar {
    max-width: none !important;
    min-width: 0px !important;
}

Unreadable input fields with dark GTK themes

When using a dark GTK theme, one might encounter Internet pages with unreadable input and text fields (e.g. text input field with white text on white background, or black text on dark background). This can happen because the site only sets either background or text color, and Firefox takes the other one from the theme. To prevent Firefox from using theme's colors in web pages confirm browser.display.use_system_colors is set to false in about:config.

Otherwise, if the previous modification didn't solve the issue, it is possible to launch Firefox with a light GTK theme by adding a new string in about:config named widget.content.gtk-theme-override and setting it to a light theme like Breeze:light or Adwaita:light.

Override input field color with CSS

Note: Related bug has been resolved starting with 68. [3]

The extension Text Contrast for Dark Themes sets the other color as needed to maintain contrast.

Alternatively set the standard colors explicitly for all web pages in userContent.css or using the stylus add-on. The style sheet is usually located in your profile folder (visit about:profiles for the path) in chrome/userContent.css, if not you can create it there.

The following sets input fields to standard black text / white background; both can be overridden by the displayed site, so that colors are seen as intended:

Note: If you want urlbar and searchbar to be white remove the two first :not css selectors.

input:not(.urlbar-input):not(.textbox-input):not(.form-control):not([type='checkbox']):not([type='radio']), textarea, select {
    -moz-appearance: none !important;
    background-color: white;
    color: black;
}

#downloads-indicator-counter {
    color: white;
}

Change the GTK theme

To force Firefox to use a light theme (e.g. Adwaita) for both web content and UI, see GTK#Themes.

Change the GTK theme for content process only

Note: Works with multiprocess enabled. [4]

To force Firefox to use a light theme (e.g. Adwaita) for web content only:

1. Open about:config in the address bar.
2. Create a new widget.content.gtk-theme-override string type entry (right mouse button > New > String).
3. Set the value to the light theme to use for rendering purposes (e.g. Adwaita:light).
4. Restart Firefox.

Web content CSS settings

This section deals with the userContent.css file in which you can add custom CSS rules for web content. Changes to this file will take effect once the browser is restarted.

This file can be used for making small fixes or to apply personal styles to frequently visited websites. Custom stylesheets for popular websites are available from sources such as userstyles.org. You can install an add-on such as superUserContent to manage themes. This add-on creates the directory chrome/userContent.css.d and applies changes to the CSS files therein when the page is refreshed.

Import other CSS files

~/.mozilla/firefox/<profile_dir>/chrome/userContent.css

@import url("./imports/some_file.css");

Block certain parts of a domain

~/.mozilla/firefox/<profile_dir>/chrome/userContent.css

@-moz-document domain(example.com) {
    div#header {
        background-image: none !important;
    } 
}

Add [pdf] after links to PDF files

~/.mozilla/firefox/<profile_dir>/chrome/userContent.css

/* add '[pdf]' next to links to PDF files */
a[href$=".pdf"]:after {
    font-size: smaller;
    content: " [pdf]";
}

Block ads

See floppymoose.com for an example of how to use userContent.css as a basic ad-blocker.

Mouse and keyboard

Mouse wheel scroll speed

To modify the default values (i.e. speed-up) of the mouse wheel scroll speed, go to about:config and search for mousewheel.acceleration. This will show the available options, modifying the following:

• Set mousewheel.acceleration.start to -1.
• Set mousewheel.acceleration.factor to the desired number (10 to 20 are common values).

Mozilla's recommendation for increasing the mousewheel scroll speed is to:

• Set mousewheel.default.delta_multiplier_y between 200 and 500 (default: 100)

Pixel-perfect trackpad scrolling

To enable one-to-one trackpad scrolling (as can be witnessed with GTK3 applications like Nautilus), set the MOZ_USE_XINPUT2=1 environment variable before starting Firefox.

If scrolling is undesirably jerky, try enabling Firefox's "Use smooth scrolling" option in Preferences > General > Browsing.

Enable touchscreen gestures

Make sure dom.w3c_touch_events.enabled is either set to 1 (enabled) or 2 (default, auto-detect).

Add MOZ_USE_XINPUT2 DEFAULT=1 to /etc/security/pam_env.conf and then logout or reboot your system for the changes to take effect.

On some devices, it may be necessary to disable xinput's touchscreen gestures by running the following:

$ xsetwacom --set device Gesture off

Mouse click on URL bar's behavior

To make the url bar behaves like in Windows regarding mouse clicks: a single click selects everything, a double click selects a single word until a punctuation sign and a triple click selects everything again, set the following in about:config:

browser.urlbar.clickSelectsAll; true
browser.urlbar.doubleClickSelectsAll; false
layout.word_select.stop_at_punctuation; true (default)

Set backspace's behavior

See Firefox#Backspace does not work as the 'Back' button.

Disable middle mouse button clipboard paste

See Firefox#Middle-click behavior.

Emacs key bindings

To have Emacs/Readline-like key bindings active in text fields, see GTK#Emacs key bindings.

Miscellaneous

Enable additional media codecs

Before continuing, remember there is a reason some of these variables are not enabled by default, e.g. stability, memory leaks, etc. Go to about:config and check the following options:

Suggested values

Key	Value	Description
media.mediasource.enabled	true (default)	Enable Media Source Extensions (MSE)
media.mediasource.mp4.enabled	true (default)	Enable MP4 MSE
media.mediasource.webm.enabled	true (default)	Enable WebM MSE.
media.mediasource.ignore_codecs	true	Enable H.264 MSE, amongst other things (This boolean key has to be created!)
media.av1.enabled	true	Enable AV1

Widevine and Netflix/Amazon Video

Widevine is a digital rights management tool that Netflix, Amazon Prime Video, and others use to protect their video content.

The first time you visit a Widevine-enabled page Firefox will display a prompt below the address bar asking for permission to install DRM. Approve this and then wait for the "Downloading" bar to disappear, you are now able to watch videos from Netflix, Amazon Prime Video, or any other Widevine protected site.

Note: Please make sure to check the Play DRM content option under Content-Preference.

It is also required that the private mode browsing is disabled, for the window and in the preferences.

Remove full screen warning

Warning about video displayed in full screen mode ("… is now fullscreen") can be disabled by setting full-screen-api.warning.timeout to 0 in about:config.

Change the order of search engines in the Firefox Search Bar

To change the order search engines are displayed in:

• Open the drop-down list of search engines and click Manage Search Engines... entry.
• Highlight the engine you want to move and use Move Up or Move Down to move it. Alternatively, you can use drag-and-drop.

"I'm Feeling Lucky" mode

Some search engines have a "feeling lucky" feature. For example, Google has "I'm Feeling Lucky", and DuckDuckGo has "I'm Feeling Ducky".

To activate them, search for keyword.url in about:config and modify its value (if any) to the URL of the search engine.

For Google, set it to:

https://www.google.com/search?btnI=I%27m+Feeling+Lucky&q=

For DuckDuckGo, set it to:

https://duckduckgo.com/?q=\

Secure DNS with DNSSEC validator

You can enable DNSSEC support for safer browsing.

Adding magnet protocol association

In about:config set network.protocol-handler.expose.magnet to false. In case it does not exist, it needs to be created, right click on a free area and select New > Boolean, input network.protocol-handler.expose.magnet and set it to false.

The next time you open a magnet link, you will be prompted with a Launch Application dialogue. From there simply select your chosen BitTorrent client. This technique can also be used with other protocols: network.protocol-handler.expose.<protocol>.

Prevent accidental closing

There are different approaches to handle this:

Under about:preferences#general select Warn you when quitting the browser.

An alternative is to add a rule in your window manager configuration file. For example in Openbox add:

 <keybind key="C-q">
   <action name="Execute">
     <execute>false</execute>
   </action>
 </keybind>

in the <keyboard> section of your ~/.config/openbox/rc.xml file.

Note: This will be effective for every application used under a graphic server.

The Disable Ctrl-Q and Cmd-Q extension can be installed to prevent unwanted closing of the browser.

Note: This extension no longer works on Linux due to a bug.

Plugins do not work with latest version

There can be compatibility issues with plugins not working with the latest Firefox version (e.g. Pentadactyl). If possible, try installing the nightly/beta builds available, or temporarily downgrade Firefox.

Disable Add-on Compatibility Checks plugin should take care of spurious compatibility issues when the plugins get disabled, even though they work just fine with the new version.

Jerky or choppy scrolling

Scrolling in Firefox can feel "jerky" or "choppy". A post on MozillaZine gives settings that work on Gentoo, but reportedly work on Arch Linux as well:

1. Set mousewheel.min_line_scroll_amount to 40
2. Set general.smoothScroll and general.smoothScroll.pages to false
3. Set image.mem.min_discard_timeout_ms to something really large such as 2100000000 but no more than 2140000000. Above that number Firefox will not accept your entry and complain with the error code: "The text you entered is not a number."
4. Set image.mem.max_decoded_image_kb to at least 512K

Now scrolling should flow smoothly.

Run Firefox inside an nspawn container

If you have problems accessing the X server from within the container, you might want to consider Systemd-nspawn#Use an X environment.

To run as PID 1

 # systemd-nspawn --setenv=DISPLAY=:0 \
              --setenv=XAUTHORITY=~/.Xauthority \
              --bind-ro=$HOME/.Xauthority:/root/.Xauthority \
              --bind=/tmp/.X11-unix \
              -D ~/containers/firefox \
              firefox

Else rather boot the container, with systemd ideally setting up your networking with systemd-networkd:

# systemd-nspawn --bind-ro=$HOME/.Xauthority:/root/.Xauthority \
              --bind=/tmp/.X11-unix \
              -D ~/containers/firefox \
              --network-veth -b

Note: There is a bug in systemd version 235 that causes /tmp/.X11-unix to disappear from the filesystem when doing this. If you are having trouble, try binding /tmp/.X11-unix read-only instead: --bind-ro=/tmp/.X11-unix

Once your container is booted, run the Xorg binary like so:

# systemd-run -M firefox --setenv=DISPLAY=:0 firefox

Disable WebRTC audio post processing

If you are using the PulseAudio module-echo-cancel, you probably do not want Firefox to do additional audio post processing.

To disable audio post processing, change the value of the following preferences to false:

• media.getusermedia.aec_enabled (Acoustic Echo Cancellation)
• media.getusermedia.agc_enabled (Automatic Gain Control)
• media.getusermedia.noise_enabled (Noise suppression)

Fido U2F authentication

Install libu2f-host for the required udev rules to allow Firefox to communicate with the U2F key.

Firefox supports the Fido U2F authentication protocol.

Note: Firefox does not inplement the entire U2F protocol [5]. Some sites might not work correctly.

Get ALSA working back

As long as Arch keeps building Firefox with ac_add_options --enable-alsa, then one can make Firefox allow ioctl syscalls, blocked by default by Firefox sandboxing, and required by ALSA setting security.sandbox.content.syscall_whitelist in about:config, to the right ioctl syscall number, which is 16 for x86-64 and 54 for x86-32. See:

[6] [7] [8]

No need for apulse if built as done so far.

See also

• MozillaZine Wiki
• about:config entries MozillaZine article
• Firefox touch-ups that might be desired