I'm running Firefox Nightly, built May 1st, 2016. For a couple days, I've been noticing that the "box" part of checkboxes isn't displaying. The control is still fully functional, in the sense that if I click on the place where it _should_ be, it shows a check, but it doesn't look right. This problem persists when I run in Safe Mode, but not in the stable channel (which I installed from distribution packages, unlike Nightly, which I installed from a Mozilla tarball). I cannot reproduce this in a separate GTK+3 application (Seahorse, to be precise). I'm running Arch Linux, fully up-to-date as of today, and GTK+ 3.20.3. I'll attach my about:support. Note that the stable Firefox which is packaged in Arch ships a libxul which is linked against libmozgtk, which is in turn linked against GTK+3.

about:support

Screenshot of Bugzilla's attachment "this is a patch" checkbox, with the checkbox in the "checked" state.

Screenshot of Bugzilla's attachment "this is a patch" checkbox in the "unchecked" state.

Correctly drawn checkboxes as seen in Seahorse

One note: GTK+3 checkboxes seem to animate now, where the checkmark is "drawn" in, similar to how one would check a box with a pencil. That may be related to this bug. Also, I'm on the default GTK+ theme (Adwaita) as well as the default icon theme. Finally, I'm running under GNOME Shell.

Thanks for your detailed report, but this is a known problem.

Whoops, sorry! I searched for duplicates, but apparently not hard enough. Thanks!