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 file.

You can also check for problems with the preferences and 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.*http://kb.mozillazine.org/Preferences_not_saved

I had this problem...again. I've tried all the solutions in this forum with minimal success. But I FOUND A SOLUTION!!

I reinstalled FF, tried fixing about:config, deleted the extensions.* files, turned off auto update on both FF and the add-ons, clicked on change add-ons to manually update, and then even went in and actually did change each one to OFF for updates. Nothing worked.

It was operator error (me). I like to do things quick so I install FF (V 24), then I open tabs for all the add-ons I know I want, then install all of them before I restart.

This time I started with a clean install, opened my add-on tabs, but installed each one, then restarted, before installing the next one.

It worked!!