Since I installed Gentoo its been bugging me that if I scrolled too quickly through a web page, it'd switch from “scrolling the page” to “next page” and flip to the page I'd used the back button to get away from. For a while I thought it was a Firefox thang, but I finally discovered that my mouse was giving different “button” messages depending how quickly I scrolled it. Normal, slow, scrolling was sending buttons “4” or “5” to the system, but quick scrolling sent buttons “6” or “7”.

A quick check of the xorg.conf file showed the mouse configuration looked like:

Section “InputDevice”
        Identifier  “Mouse0”
        Driver      “mouse”
        Option      “Protocol” “IMPS/2”
        Option      “Device” “/dev/input/mice”
        Option      “ZAxisMapping” “4 5 6 7”
EndSection

Reducing that ZAxisMapping option to just “4 5” seems to have cured things.

“You can select columns, rows, or blocks of cells in HTML tables in Firefox (or any Gecko-powered browser) by holding down "Control" and selecting with the mouse. Handy for copy pasting data from tables in websites.”

Selecting Columns and Rows in HTML Tables with Firefox | Acts of Volition