You can try re-building the list of installed add-ons. First you need to open the profile folder (location where user data is stored). In the menus select "Help > Troubleshooting Information" In the troubleshooting information tab, click on the "Open Containing Folder" button. This should open the profile folder in Windows Explorer (or your default file manager if it is not set to Windows Explorer) Now close Firefox. In Windows Explorer, rename the following files: extensions.ini to extensions.ini.old extensions.cache to extensions.cache.old extensions.rdf to extensions.rdf.old compatibility.ini to compatibility.ini.old If you have a file called extensions.sqlite, delete it. Re-start Firefox, the first 4 files will be re-created and in the process it re-builds the list on installed add-ons, and hopefully this will resolve your problem.