Sent to you by Vihang Shah via Google Reader:
Firefox 3's new and improved fullscreen browsing introduces a truly fullscreen experience, utilizing every last pixel of your monitor for the web page. By default the new fullscreen autohides any visible toolbars, though you can bring them back at any time by hovering your mouse over the top edge of the screen. Pre-Firefox 3 extensions introduced similar functionality, like the previously mentioned Autohide extension, so this is just one more built-in Firefox 3 feature that allows you to ditch your Firefox 2 extensions. If you don't like the new behavior, a couple of small
about:config tweaks, as detailed in the Mozilla Links post, will get you back to the old-style fullscreen.