The Problem

In the newer versions of Firefox (0.9.3), mouse buttons 6 and 7 are
bound to history forward and back. This is really inconvenient
when using a synaptics touchpad driver that generates those
buttons for horizontal scrolling across the bottom.

The fix

Found this via Google "firefox mousewheel horizscroll" at:

• http://www.antrix.net/stuff/suse_thinkpad/
• http://subgeek.net/archives/raves.html
• http://www.macosxhints.com/article.php?story=20040617000005758

Go to about:config and change these settings:

mousewheel.horizscroll.withnokey.action => 0
mousewheel.horizscroll.withnokey.sysnumlines => true