I use the third mouse button to close tabs in Firefox and almost anything else with tabs without having to press the X button. I've grown quite used to using it for almost everything now. Mostly games, though. I also appreciate the left and right scroll button toggles (that is, you can move the actual scroll wheel left or right and it triggers two separate click actions apart from the actual middle mouse click or scroll-wheel). Handy for back and forward buttons while browsing.