Adding Firefox Search Engines As User
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.