Getting fourth and fifth mouse button to work in browsers

You could always just install Steermouse or USB Overdrive (Google 'em); they both allow you to map mouse buttons to do pretty much whatever action you want. Otherwise, see if your mouse came with any drivers that would let you do the same thing (Kensington Mouseworks, Logitech Control Center, etc. depending on your mouse brand).

 

I have the same mouse and use USB Overdrive, it works well but since I haven't payed for it I always have this annoying popup when I start the computer. Oh well better than spending 30 bucks to have 2 extra buttons.

If the authors put more reasonable prices (less for something as simple as this), aka 5$, I would no doubt pay.