Adding Firefox Search Engines As User
Before Firefox 1.5
Firefox writes search engine files to /opt/mozilla/lib/firefox/searchplugins, which is, by default, readable only by root. This means users cannot add search engines. You can either
chmod o+w this folder, or run firefox as root when you want to install a search engine.
To remove search engines, remove the appropriate search engine files in /opt/mozilla/lib/firefox/searchplugins. Next time you start Firefox the engine won't be in the list.
Since Firefox 1.5
Search engines could now be added in the user's profile. If you want to get a look : ~/.mozilla/firefox/xxx.default/searchplugins where xxx is you profile id).
There is also a plugin for Firefox which makes this very easy. It is called OpenSearchFox. Once installed, to add a search engine just right click on the search field and follow the wizard.