3 suggestions/Wishes

[Katana Man]

The latest drivers didn't help. I'm using a MouseMan Wheel optical USB mouse.

In the control panel, I can define many different commands/keys for any of the buttons. It will work if I set the thumb button to become Backspace. But Backspace is more dangerous than Internet Back.

I use FlashFXP on a daily basis, and I'm almost willing to get a Microsoft Mouse just to make this work. But this mouse is just so comfortable
http://www.tweakmax.com/html/logitechopt/optical-1.cfm

and Microsoft mouse buttons are usually set too light, so that just resting your fingers on them triggers the button.