For those needing more screen space, starting with the Beta 5 Release of Firefox 3.6, Windows users can now hide the Menu Bar. To hide the Menu Bar, right-click on an empty area in the toolbars and uncheck Menu Bar to hide. If you need to access the menu bar, simply press the ALT key and the menu bar will re-appear temporarily.
Firefox 3.5 users can also hide their Menu Bar, but will need an extension such as Personal Menu or Compact Menu 2 (note: I have not used either of these extensions so I can not say if one is better than the other.)
Source: Mozilla Links