Updated to Firefox 3.6 today. As usual, a very solid update, and was pleasantly surprised by the full-screen mode (finally!) that works exactly as you would want it. This was one of the few areas where Firefox on OSX was lagging behind as compared to other platforms.
There were a few plugins that claimed to enable this in prior versions, but never worked very well.
The option is available under the “View”menu and has a standard Apple-Shift-F shortcut as well.