On X11, with MOZ_USE_XINPUT2=1 set in the environment Firefox exhibits much
improved pixel-by-pixel touchpad scrolling, and touchscreen scrolling actually
works. On Wayland, the behavior is already enabled by default so the
environment variable has no effect.

Historically, the reason why this was not set by default on X11 was an
outstanding bug that caused scrolling with wheel mice to periodically break:
https://bugzilla.mozilla.org/show_bug.cgi?id=1182700

However this bug appears to only affect KDE Plasma systems, and the KWin
developers just fixed an issue in KWin, and now the problem no longer
manifests. See https://bugs.kde.org/show_bug.cgi?id=394772.

Therefore, there no longer appear to be any blockers to enabling this behavior
by default by setting MOZ_USE_XINPUT2=1 in the environment. X11 Firefox users
should really appreciate this change.