It is possible that there is a problem with the file(s) that store the extensions registry. Delete the extensions.* files (e.g. extensions.json, extensions.sqlite, extensions.ini) 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 or not compatible extensions in "Firefox/Tools > Add-ons > Extensions" then click the Tools button at the left end of the Search Bar to check if there is a compatibility update available. If this hasn't helped then also delete the addons.json and addons.sqlite files. You can check for problems with preferences. Delete a possible user.js file and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release. *http://kb.mozillazine.org/Preferences_not_saved *http://kb.mozillazine.org/Resetting_preferences

Thank you for your help. The only file I saw to delete was extensions.json (no .sqlite or .ini files), so I just deleted that one. Once I restarted Firefox, it prompted me to re-install all my add-ons, and everything is working fine now.