this sounds like the files in your [[Profiles|profile folder]] that store the information about your extensions might have become corrupted. please go to firefox > help > troubleshooting information > profile - show folder... then windows explorer should open up your profile folder. in there look for the files named extensions.ini, extensions.sqlite, extensions.sqlite-journal (and in some cases also extensions.txt, extensions.rdf). delete or rename those files and quit firefox, they will be regenerated the next time you launch firefox. also see [https://support.mozilla.org/en-US/kb/Unable%20to%20install%20add-ons#w_corrupt-extension-files Corrupt extension files]