My xorg.conf let me down after my upgrade to FC5. I have a Logitech USB MX310 and this is how I got my back/forward buttons to work in Firefox.
/etc/X11/xorg.conf:
Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "ExplorerPS/2"
Option "Device" "/dev/input/mice"
Option "Buttons" "7"
Option "ZAxisMapping" "4 5"
Option "ButtonMapping" "1 2 3 6 7"
Option "Emulate3Buttons" "no"
EndSection
And remove any pointer declarations from ~/.Xmodmap and /etc/X11/Xmodmap.