You can delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox [[Profiles|profile folder]] to reset the extensions registry. New files will be created when required. See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions If you see disabled extensions that are not compatible on the next start in "Tools > Add-ons > Extensions" then click the "Find Updates" button to do a compatibility check.