This extension enhances the full screen mode into a really full screen mode, hiding the remaining tool bars and status bar and making them visible again when the mouse pointer hits an edge of the screen.
This is a really nice little enhancement to the full screen mode of Firefox. There is an icon on the status bar to go to full screen or you can use default full-screen key F11. Press F11 again to return to regular mode. Highly recommended. Download it here.