On my ThinkPad, I scroll by holding down the middle mouse button and moving the little trackpoint joystick. This means that it is easy to couple a little bit of sideways scroll with vertical scroll, which is not usually a problem. In Firefox, however, the default settings are for a left/right mouse scroll to go back/forward a page. This is most unsatisfying.
The solution has been published online in a multitude of places, but to save me needing to search for it again I’ll mention it here. Open a new tab and type about:config into the address bar. Type mousewheel.horizscroll.withnokey into the “Filter” bar that appears at the top of the page to find the necessary entries.
Make the following changes by double-clicking the appropriate option in the list:
mousewheel.horizscroll.withnokey.action => 0
mousewheel.horizscroll.withnokey.sysnumlines => true