Hide Menubar supports to hide the Menu Bar automatically, you can press the ALT key to show it temporarily.

FAQ:
Q: I have installed HideMenubar 3.6, and restarted Firefox, but menubar is still there, it seems not working?
A: Right-click on menubar -> Uncheck the "Menu Bar".

The license of this software is Freeware, you can free download and free use this browser plug-ins software.