So this must be a new feature of Firefox (3.6.3)- but you can now browse completely full screen with everything disappearing but the page. And when you scroll to the top, your browser bar and tabs reappear sans the OS X menu bar and window bar. It’s very nifty I must say. While full screen is nothing new really, it’s cool that Fx now does it and eliminates any unnecessary items like Safari on the iPhone does. It basically makes anything else disappear and declutter your browser experience. Full screen mode is selectable in the menu bar or by pression CMD+Shift+F.
(Of note- Chrome also does this although I don’t like it’s execution as much as Fx’s because cursoring to the top of the page makes the menu bar reappear and entering/exiting FS mode causes the screen to flash black which is obscenely annoying.)