FullerScreen is a Firefox extension that will really enhance your browsing experience. Firefox has a built-in full screen feature, but it will still display tabs and toolbars. The Firefox full screen feature can be accessed by hitting F11.
However, if you want to experience true full screen browsing, you can install the Firefox FullerScreen Add-on. What FullerScreen does is hide the tabs and toolbars until you request it by moving your mouse to the top. FullerScreen Add-on can be found at the Firefox Add-On Directory.
Below are screenshots of Firefox with and without the FullerScreen Addon: