Every browser that I know of already lets you use the Backspace key to navigate back. Also ctrl-w seems to be the default for closing the current tab (among other things in other programs).
I've never really had a use to navigate forward, but my All In One Mouse Gestures for Firefox makes that easy, allowing to merely right-click and drag to the right to go forward. Of course, that kind of defeats the purpose of not using a mouse at all, but it still makes it easier on laptop users having to use that stupid touchpad.