In Firefox 3 on Linux, the mouse button assignments for Back and Forward have changed, to follow X11 customs. Previous versions of Firefox used buttons 6 and 7 for Back and Forward, respectively. Firefox 3 uses buttons 8 and 9. You can configure Firefox to use 6 and 7 by editing some hidden preferences.
In the address bar, type about:config and press EnterReturn.
The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!I accept the risk! to continue to the about:config page.
In the Filter: box, type mousewheel.horizscroll.withnokey.
Double-click on mousewheel.horizscroll.withnokey.action, set the value to 2, and click OK.
Double-click on mousewheel.horizscroll.withnokey.numlines, set the value to -1, and click OK.
Double-click on mousewheel.horizscroll.withnokey.sysnumlines, which should set the value to false.