Firefox is interfering with KWin after an update in openSUSE Tumbleweed roughly a few hours ago. All other applications (including Thunderbird) work fine, but when I startup Firefox its window has no borders whereas the titlebar appears gray. The window can also not be resized, it can only be minimized / restored by dragging it from and to the screen edges.

I'm attaching a screenshot of the broken FF window. The gray titlebar is NOT the theme I'm currently using (Aurorae), and I have no idea what skin the buttons to its upper-right are from either. KWin should not be allowing applications to have custom window managers, this might be an issue with some fundamental library.