Leap 42.2 up-to-date
Running an up-to-date version of Firefox, I do a few screen shots with Spectacle, save and exit each instance, then Spectacle shows a pop up frame just above the panel and icons on the panel are no longer clickable. It is possible to access running apps and Firefox frames using Alt/Tab, but not possible to control apps from the panel -- all icons on the panel are frozen. Also the digital clock stops advancing at this point. If KSysGuard were running before the freeze, it would be possible to kill Firefox, but if not, it is necessary to move through the open Firefox frames, closing each one.