Configuration and add-ons reside in %AppData%, or an orherwise user-writable profile directory. Compromising the executable, which lives under %ProgramFiles%, or an otherwise protected directory, takes administrator rights.
Beyond this plausible inconvenience, however, Mozilla simply doesn't want regular users messing with unapproved add-ons. Just switch Firefox to Developer Edition for that. It's been very stable, in my experience.