There’s nothing like a little more reading space, is there? If you leave your mouse alone for a while these buttons get the heck out of your way. They slide back up when you move your mouse.
This is even cooler if you use our handy spacebar shortcut (You remember that you can move forward using the handy spacebar key, right?). This newfangled buttonbar fanciness works in Firefox and Safari, so read comfortably, with all the room you crave.