Firefox: Enabling pixel scrolling with touchpads.

There is a [bug in Firefox on Ubuntu](https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1697122)
that prevents precise scrolling when using a touchpad or a trackpoint.
One way to fix[1] this is to start Firefox with `MOZ_USE_XINPUT2=1 firefox`, or put

    MOZ_USE_XINPUT2=1

in `/etc/environment`.

[1] In fact this may trigger [other bugs](https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1697122/comments/10) when a notification appears while scrolling.

tags: computers, documentation, ubuntu