I have a Microsoft Comfort Optical Mouse 3000 v1.0 which plugs in via USB. When I hit the back button on my mouse in firefox nothing happens.

If the mouse generates an output in xev, you can map that keycode in your keyoard shortcuts.

xev says the back button is button 9, how do I map that keycode in my keyboard shortcuts?

Keyboard shortcuts can be set in System -> Preferences -> Keyboard Shortcuts. If your multimedia keys don't work with that, try the 'keytouch' package, explained at http://keytouch.sourceforge.net - See !Keyboard for changing layouts

Sorry, didn't help.

Is there a way to edit xorg.conf to set this?

Right now button 9 is set to forward actually, but I want it set to back.

No, shortcuts are not defined in xorg.conf. If the mouse is working and all buttons are sending input then xorg is doing its job. It is down to higher configs to managethis input.

Try this:

http://ubuntuforums.org/showthread.php?t=350411

May help

How do I make xmodmap remap button 9 to button 8 on my mouse? Or button 9 to button 10?

Figured it out:

xmodmap -e "pointer = 1 2 3 4 5 6 7 9 8"

However how do I make that get executed whenever a user logs into GNOME? (IE for all users).

I think it's /etc/X11/Xmodmap actually.

Yup, all good now.