Computer Help and Support

You would need to delete your firefox profile and start again, this means losing bookmarks etc.

You could grab a copy of MozBackup http://mozbackup.jasnapaka.com/
Then back up the stuff you want to keep, ie bookmarks, passwords etc but not the extensions. (There is an option for this during backup).

Then I'd get Revo Uninstaller https://www.revouninstaller.com/revo_uninstaller_free_download.html
And completely remove the old firefox. Make sure you do a restore point first, and select the highest level of uninstall and scan afterwards to delete all vestiges of the profile.

After that, install new Firefox. Start it once to create a new profile, then close it down. Using MozBackup, restore your old profile minus the extensions.

On edit:
Not sure how conversant you are with the Firefox profiles, but they are stored in Documents & Settings, not the Firefox program folder. You'll need to turn on hidden files and folders to access it.