Firefox 3 has improved fullscreen mode to the point where it is almost fullscreen. All that remains is the toolbar and tabbar at the top being hidden away and ready to appear when you move your mouse to the top of the screen. Unfortunately this takes up 5 pixels of the screen height and I was looking for a way to get rid of that too to really use the entire screen.
It took a while to guess the right search term to find what I needed but I finally did. The Full Fullscreen add-on enables the desired behaviour. All you have to do is enable the "Prevent navbar auto-popup" option in its preferences dialog.