The MOZ_USE_XINPUT2=1 version brings real smooth scrolling and touchscreen input, but is barely known.
It should be enabled by default, there's no drawback.
The simplest solution could be to create /etc/env.d/80firefox with MOZ_USE_XINPUT2=1
...i'm going to sit on this one and think a while. I would generally prefer not to install env.d/ entries for things like this, and instead leave it up to the user to do in their own profiles. That said there may be build-time options or 'pref' options that can be leveraged to take care of this, too.
FYI, this is the upstream tracker bug related to enabling xinput2 by default: