This page contains advanced Firefox configuration options and performance tweaks.



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.

Advanced Firefox options

This section contains advanced Firefox options for performance tweaking. For additional information see these Mozillazine forum posts.

Enable OpenGL Off-Main-Thread Compositing (OMTC)

Warning: 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.
Note: Since Firefox version 40 basic software OMTC is enabled by default except on machines mentioned above.

To enable OpenGL OMTC go to about:config and set:

  • layers.acceleration.force-enabled true

Restart Firefox for changes to take effect.

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 the above changes do not enable GPU acceleration, try setting the environment variable as follows: export MOZ_USE_OMTC=1. Then run Firefox [1].

For more information on OMTC in Firefox read here:

Set AzureContentBackend to Skia instead of Cairo

Note: Since Firefox 51 skia is the default content backend

Skia is a 2D open-source graphics library to eventually supersede Cairo as the default Azure backend on Linux.

To set Skia as the default go to about:config and set:

  • skia,cairo

Restart Firefox for changes to take effect.

To confirm the default Azure backend go to about:support and under the "Graphics" section look for "AzureContentBackend" and check if it reports "skia".

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 and set it to true.

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

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 OCSP validation

OCSP validation may cause Firefox to become slower for each (HTTPS) connection to a new server. This is worse, recently, where web gadgets are included in pages via HTTPS (e.g., "like" buttons of the social networks), resulting in many connections for a single URL.

  • Turn off the following option under Preferences -> Advanced -> Certificates -> "Requests: Use the Online Certificate Status Protocol (OCSP) to confirm the current validity of certificates".
Warning: Disabling OCSP causes vulnerabilities to man-in-the-middle attacks. If you are using a potentially vulnerable connection such as Wi-Fi or VPN, it is strongly advised to leave OCSP on.

Turn off the disk cache

Every object loaded (html pages, jpeg images, css stylesheets, gif banners) is saved in the Firefox cache, to be loaded in the future without to download it again from the server, but only fraction of these objects will be really reused without download (usually the 30%). This because of too short expiration times for the objects, updates or simply the user behavior (to load new pages instead the ones already visited). The Firefox cache is divided in memory and disk cache and using the disk cache results to frequent disk writes, because every time an object loaded it is written to the disk and some older object is removed.

  • Turn on the following option under Preferences -> Advanced -> Network -> "Cached Web Content: Override automatic cache management" and specify zero in "Limit cache to".

Longer interval to save session

The Firefox session store automatically saves the current status (opened urls, cookies, history and bookmarks) to the disk every 15 seconds. It may be too frequent for the user needs, resulting in a frequent disk access.

This setting can be found on the about:config page (try searching for sessionstore).

  • browser.sessionstore.interval 300000

If you want to disable this feature, then you will need to change the following setting from true to false.

  • browser.sessionstore.resume_from_crash false

Immediate rendering of pages

Mozilla applications render web pages incrementally - they display what has been received of a page before the entire page has been downloaded. Since the start of a web page normally does not have much useful information to display, Mozilla applications will wait a short interval before first rendering a page. This preference controls that interval. Note that if you are on slower connections (dial up) changing this setting might make web pages load for longer times even though the page appears faster.

This setting can be created in the about:config page as

  • nglayout.initialpaint.delay with a value of 0.

Referer header control

Firefox as of version 28 has the ability to control how the HTTP Referer header is send. The options are hidden from the GUI configuration, but available through about:config. There are four related keys. The source post mentions recommended and default settings. A short description of the recommended setting is given in table, as well as the values to achieve a perfect score on

Suggested values
Key Recommended Default Description
network.http.sendRefererHeader 1 1 2 Only for clicked links (0 disabled completely, 1 send referer on link click, 2 always send; incl. images)
network.http.referer.XOriginPolicy 1 1 0 Only if base domains match (0 always send, 2 only if host matches)
network.http.referer.spoofSource true false false Send target URI instead
network.http.referer.trimmingPolicy 2 0 0 scheme, host and port (0 full; 1 scheme, host, port and path)

Defragment the profile's SQLite databases

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

Starting with Firefox 3.0, bookmarks, history, passwords are kept 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-cleanerAUR 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 %

Recent (as of H2 2016) versions of Firefox, provide 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. In the resulting 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.

Turn off sponsored content and tiles

In about:config, set the string value to a blank for both of these: and Consider also disabling the tile feature from the tools on a new tab page. A Wireshark session demonstrates the level of chatter created by these features.

Enable Electrolysis

In firefox 48 (45 ESR) or later, Electrolysis (multi-process) may be enabled to improve performance and security by setting browser.tabs.remote.autostart to true in about:config. It may be needed to force-enable Electrolysis [2], although this is generally not recommended and may cause issues.

To check if Electrolysis is enabled, go to about:support and under the "Application Basics" section look for "Multiprocess Windows". If it reports "0/1 (Disabled)", Electrolysis is disabled; if it reports "1/1 (Enabled by user)" it is enabled. Note that the given numbers */* indicate the number of open Firefox windows, e.g. 0/2 meaning non of the two Firefox-windows are using Electrolysis, and 2/2 means it is enabled for both windows.

Enable HTTP Cache

In about:config, set browser.cache.use_new_backend from value 0 to 1.

Disable Pocket

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

Note: on 45ESR the keys are browser.pocket...



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 [3] 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, 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.

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

Change the font

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

* {
    font-family: "FONT_NAME";

Hide button icons

Enables text-only buttons:

.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.

/* 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:

/* 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 window border and title bar

Install the Hide Caption Titlebar Plus extension and set the following settings (leaving other settings to default):

Option Value
Show Custom Caption/TitleBar Never
Activate custom borders and corner resizers Deactivate
Enable Customizable Buttons (min,max,close) Using a Glass-like window background
Custom Minimize, Max, Close Buttons Auto. Current theme's skin (fixed position)
Drag Fx window using Tab-bar background Enable
Alternative hide-titlebar feature Use

The extension Classic Theme Restorer provides more tweaking options to get the best result (e.g. set tab height to 28px, enable/disable toolbars/buttons, etc.).

Auto-hide Bookmarks Toolbar

#PersonalToolbar {
    visibility: collapse !important;

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

Remove sidebar width restrictions

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

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 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

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

Block certain parts of a domain

@-moz-document domain( {
    div#header {
        background-image: none !important;

Add [pdf] after links to PDF files

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

Firefox 4 New Menu Bar/Firefox Button

To toggle between the new Firefox button and the classic menu bar:

  • if the button is active, check Preferences > Menu Bar, or right click in the toolbar area and check Menu Bar.
  • if the menu bar is active, uncheck View > Toolbars > Menu Bar, or right click in the toolbar area and uncheck Menu Bar.

In GNU/Linux, you will just get a plain grey button instead of the new orange one from Windows. However you can change this to either a Firefox icon or the icon followed by the "Firefox" text.

Adding the following to your ~/.mozilla/firefox/userprofile/chrome/userChrome.css file will place the icon before the text:

#appmenu-toolbar-button {
  list-style-image: url("chrome://branding/content/icon16.png");

Adding the following to the same file will remove the "Firefox" text:

#appmenu-toolbar-button > .toolbarbutton-text,
#appmenu-toolbar-button > .toolbarbutton-menu-dropmarker {
  display: none !important;

This userChrome.css configuration copies the default Windows Firefox 4+ look and adds an orange background to the button, with a purple background in Private Browsing mode:

#main-window:not([privatebrowsingmode]) #appmenu-toolbar-button {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(34,85%,60%), hsl(26,72%,53%) 95%) !important;
    border: 1px solid #000000 !important;

#main-window:not([privatebrowsingmode]) #appmenu-toolbar-button:hover:not(:active):not([open]) {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(26,72%,53%), hsl(34,85%,60%) 95%) !important;
    border: 1px solid #000000 !important;

#main-window:not([privatebrowsingmode]) #appmenu-toolbar-button:hover:active,
#main-window:not([privatebrowsingmode]) #appmenu-toolbar-button[open] {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(26,72%,53%), hsl(26,72%,53%) 95%) !important;
    border: 1px solid #000000 !important;

#appmenu-toolbar-button {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(279,70%,46%), hsl(276,75%,38%) 95%) !important;
    border: 1px solid #000000 !important;

#main-window #appmenu-toolbar-button:hover:not(:active):not([open]) {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(276,75%,38%), hsl(279,70%,46%) 95%) !important;
    border: 1px solid #000000 !important;

#main-window #appmenu-toolbar-button:hover:active,
#main-window #appmenu-toolbar-button[open] {
    -moz-appearance: none !important;
    color: #FEEDFC !important;
    background: -moz-linear-gradient(hsl(276,75%,38%), hsl(276,75%,38%) 95%) !important;
    border: 1px solid #000000 !important;
Note: You need to create both the chrome directory and userChrome.css, if they do not already exist.

Block ads

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

Remove fullscreen 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


Other tips and tweaks.

Enable additional media codecs

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: This section is not completed, there are many other variables related to Media Codecs in firefox. (Discuss in Talk:Firefox/Tweaks#)

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!)

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.

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).

Alternatively you can install the SmoothWheel add-on.

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 "Smooth Scrolling" option in Preferences > Advanced.

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.

How to open a *.doc automatically with Abiword or LibreOffice Writer

Go to Preferences > Applications and search for Word Document (or Word 2007 Document for *.docx). After finding it, click the drop-down list and select Use other.... From there you have to specify the exact path to the Abiword or Writer executable (i.e./usr/bin/abiword or /usr/bin/lowriter).

"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:

  1. Type about:config in the address bar.
  2. Search for the string keyword.url.
  3. Modify its value (if any) to the URL of the search engine.

For Google, set it to:

For DuckDuckGo, set it to:\

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.

The next time you open a magnet link, you will be prompted with a Launch Application dialogue. From there simply select your chosen torrent client. This technique can also be used with other protocols.

Adding magnet protocol association for kTorrent (KDE4)

Create a user copy of /usr/share/applications/kde4/ktorrent.desktop to ~/.local/share/applications/kde4/, and append to Mimetype=


Modify ~/.config/mimeapps.list and append:


See [4]

Prevent accidental closing

The Disable Ctrl-Q Shortcut extension can be installed to prevent unwanted closing of 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">

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

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

Plugins do not work with latest version

Due to Arch's bleeding edge nature, there can be some compatibility issues with plugins not working with the latest Firefox install (e.g. Pentadactyl). If possible, try installing the nightly/beta builds available, or see Downgrading packages.

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

Tango-go-next.pngThis article or section is a candidate for moving to systemd-nspawn.Tango-go-next.png

Notes: This is potentially useful for any browser, so... move back? (Discuss in Talk:Firefox/Tweaks#)

Note: on newer systems, you must enable local access to the user xserver with 'xhost local:root'

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 \

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

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

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

Show search matches position in scroll bar

This chrome feature can be achieved via FindBar Tweak extension.

Enable touchscreen gestures

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

Run export MOZ_USE_XINPUT2=1 before launching Firefox. To make this change persistent, add that command to /etc/profile.d/

Disable WebRTC audio post processing

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

To disable audio post processing, set the following parameters
Key Value Description
media.getusermedia.aec_enabled false AEC = Acoustic Echo Cancellation
media.getusermedia.agc_enabled false AGC = Automatic Gain Control
media.getusermedia.noise_enabled false Noise suppression. BEWARE: Disabling noise suppression in Firefox may lead to a "Mickey Mouse voice"

Make URL bar behave like Firefox on Windows regarding mouse clicks

In about:config, set the following settings:

browser.urlbar.clickSelectsAll true
browser.urlbar.doubleClickSelectsAll false
layout.word_select.stop_at_punctuation true

This will make a single click in the URL bar select everything, a double click selects a single word until a punctuation and a triple click selects everything again.

See also