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. You can use this button to go to the currently used Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) 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 possible addons.json and addons.sqlite files. ---- Please update to the current Firefox 31 release. *Help > About Firefox *https://support.mozilla.org/kb/update-firefox-latest-version. *https://support.mozilla.org/kb/Updating+Firefox The Firefox version that you currently run is no longer supported with security updates. *It is important to update Firefox and add-ons to the latest version to get all security fixes.