Review Mozilla 1649579: Clipboard intermittent clear bug when closing firefox

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1649579

Probably our patch is active also on Linux by default, so probably not really something to worry about (and as a plus, it might be possible to keep the clipboard data when closing the browser, if we define the pref not to empty it 🎉).

The commit itself is much more interesting than the Bug: 380f88e2

/cc @ma1