It is possible that there is a problem with the file(s) that store the extensions registry. Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache) and compatibility.ini in the Firefox profile folder to reset the extensions registry. *https://support.mozilla.org/kb/Profiles New files will be created when required. See "Corrupt extension files": *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.org/kb/Unable+to+install+add-ons If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to check if there is a compatibility update available. If this hasn't helped then also try to delete the addons.sqlite file. You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values. *http://kb.mozillazine.org/Resetting_preferences *http://kb.mozillazine.org/Preferences_not_saved