Quoting from https://bugzilla.mozilla.org/show_bug.cgi?id=1390795#c1 We explicitly keep MOZ_USE_XINPUT2 off by default because behaviour is buggy when it is turned on. The code is at http://searchfox.org/mozilla-central/rev/13148faaa91a1c823a7d68563d9995480e714979/toolkit/xre/nsAppRunner.cpp#3850 and references a couple of bugs. From the discussion in those bugs it's still not clear to me whether this needs to be fixed in Firefox or elsewhere. Some of the comments seem to imply it affects other apps as well, not just Firefox, which would make it a GTK issue most likely. But I'm not sure.