Real Fullscreen is a simple Firefox add-on that allows you a true full-screen web browsing experience.
Firefox comes with a full-screen view already, but it actually leaves a gray bar at the top of the screen. If that annoys you, Real Fullscreen can remove it really easily.
Install Real Fullscreen then, once Firefox has restarted, hit F11 to pop you into full-screen mode. Unlike the standard Firefox view, if you mouse over the top of the screen, the toolbar will not appear. Hitting F11 again returns you to windowed mode.
Not being able to access the toolbar is the only problem with Real Fullscreen. Otherwise, the clean full screen view is somehow better than the standard version, and is a totally unobtrusive add-on, as it doesn't add any logos or buttons to the interface.
Real Fullscreen is just what it says it is, and is a simple way to get a 'true' full-screen view while browsing the web.