I am sure you have probably tried this but have you tried hitting F11? F11 toggles firefox's fullscreen mode but I still have a menu bar when I do that so that might not be the problem. You could uninstall and reinstall firefox if that doesn't help, I am not sure if that would delete your settings or not as your settings are located in:
Code:
C:\users\{USERNAME}\AppData\Local\Mozilla\Firefox
On Windows 7, you could try to delete that folder if you want but that would erase your bookmarks and profile.
Thats all the help I can really offer, sry.