Files that store information about the extensions you have installed can sometimes be corrupted. Deleting them can fix a number of extension issues; they will be regenerated the next time you start Firefox.
Exit Firefox completely
open your Firefox profile folder (read the linked article for its location) and delete* these three files:
Note : Although unwanted files/folders can be deleted, renaming them is generally considered a safer option. This achieves the same result, but allows the user to retrieve any possibly required information from them later on. In the case above, rename the above 3 files as , ‘extensionsOLD.ini’, ‘extensionsOLD.cache’, ‘extensionsOLD.rdf’