Try deleting the files that store details of installed add-ons, if one of them is corrupt it can cause problems with add-ons. The files will be re-created when you re-start Firefox, and in the process will rebuild the list of installed add-ons. # Open your profile folder in Windows Explorer, for details of how to find it see [[profiles]] # Close Firefox # In the profile folder delete extensions.rdf, extensions.cache and extensions.ini # re-start Firefox to see if add-ons are working again If that does not help, do you get any error messages?