If you don't mind losing bookmarks, extensions, etc then do start > run > firefox.exe -ProfileManager and then delete all profiles and make a new one, set it as the default and away you go.
EDIT: Can back up your bookmarks by doing start > run > %AppData%\Mozilla\Firefox\Profiles\ picking the profile folder and copying out the bookmarks.html file, which can be re-imported when you have a working profile.
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