Recent Firefox 3 nightlies finally offer true full screen mode. In Firefox 2 (and previous Firefox 3 betas including beta 4) when you press F11 or select Full Screen from the View menu, Firefox expands to use all the screen but still shows the navigation and tab bars. To effectively use all available screen state you need an extension like FullerScreen.
Now, in full screen mode, the actual page contents use the whole screen estate with neither the navigation bar or the tab bar on sight. Instead, they slide to the upper edge as a hint of what happened to them for first time users. When you move the mouse cursor to the top the bars are shown again. A right click on the navigation bar shows a small menu to make the toolbars visible or exit full screen mode.
Even better, web developers can know if full screen mode is active or not through a new interface and could adapt the user interface accordingly.