Firefox 3.6 beta 5 features a new option to hide the menu bar. Right click on the navigation or menu bar and uncheck the Menu Bar item to have it gone for good.
With today’s current resolutions, you will instantly get somewhere between 20 – 40K extra pixels.
When you need to access a menu bar option, press the Alt key and the menu bar will appear temporarily.
To do the same in Firefox 3.5 you will need an extension like Personal Menu or Compact Menu 2.