It is possible that there is a problem with the file(s) that store the extensions registry. Delete t

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 do a compatibility check or see if there is a compatibility update available.

You may have to reset the preferences as well.

http://kb.mozillazine.org/Resetting_preferences

https://support.mozilla.org/kb/Resetting+preferences