Comment # 1 on bug 1032003 from Nathaniel Graham

After a few weeks with MOZ_USE_XINPUT2=1, it seems to have some pretty
significant limitations. Very frequently (> 10x/day), scrolling with a scroll
wheel will break for a while, and scrolling with two fingers on the touchpad
will become interminably slow. This is a really bad user experience.

Other distros like Fedora package a version of Firefox where scrolling works
fine out of the box for both a touchpad and scroll wheel, so it's definitely
possible, somehow.