The following information was snagged from http://gentoo-wiki.com/HARDWARE_Synaptics_Touchpad#Horizontal_Scroll_Issues_with_Firefox
I was looking for this information for a long time and couldn't find it. I finally found reference to this in a cached version of this (gentoo-wiki).
So that firefox will not misinterpret the horizontal scroll as 'back' and 'forward' For many like me this is irritating because you are reading a webpage and by moving the mouse you accidently go to another page.
Some forums suggest disabling horizontal scroll (editing xorg.conf to: Option "HorizScrollDelta" "0"), but instead the correct way is to configure firefox so that it doesn't misinterpret the horizontal scroll. In firefox type in URL (address bar):
and double-click on the line:
to set it to 0. And then also set