After almost a decade of building Firefox from source I finally bit the bullet to switch to a precompiled binary last month. It's fixed all the issues that have started to crop up but leaves a bad taste in the mouth.
I had been using their binaries due to security updates and having to build at inopportune times for an hour or three (depending on machine), but like I wrote in a sibling comment, enabling the cairo-gtk2 backend resulted in crashes when trying to select a file with the gtk file dialog. So I bit the bullet and am struggling with the drawing regressions of GTK3.
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