I use an Intellimouse Explorer 4.0A, which has always required modifications of xorg.conf for forward/back to work under linux.

With firefox 2.0, button mappings for forward and back required one of two things to make them work:

Option "Buttons" "5"
Option "ZAxisMapping" "4 5"
Option "ButtonMapping" "1 2 3 6 7"

or

xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7 10 11"

In firefox 3.0 (at least beta4), button mappings have changed. To get my my buttons to work, I had to run:

xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7 10 11"

which simply undid the change I had made to xorg.conf for firefox 2.0.

I have now removed my mouse-related modifications to xorg.conf.