That's interesting. I've been happily using firefox-gtk2 and it's been working quite reliably.

Not only I haven't had any issues with it so far, it even looks better than Arch's official package (that's built with gtk3).

The issues have been present with gtk >= 3.20, and they should be fixed with release 50 and 51:

https://bugzilla.mozilla.org/show_bug.cgi?id=1266914

https://bugzilla.mozilla.org/show_bug.cgi?id=1264079