I was waiting to see if the author of the contrib would jump in, but I guess not. (Contribs are contributed, as the name suggests, and we can't necessarily support all of them directly.)

That said, browsers are likely to be a specific screw case these days because sandboxing means that windows or even individual tabs may be running in their own processes (they certainly are in Chrome / chromium). It is not reliably possible (indeed, not necessarily possible at all) to go from an X11 window to all the associated processes, especially going in both directions (that is, downward from a window to tab sandbox processes *and* up from it to the parent browser process(es)), so stopping the whole browser is not going to happen and individual tab processes may well keep running as well, with only the process that created the window itself stopped. (I can't speak for Firefox, but for Chrome this will be a process largely independent of all the tabs, other windows, and various other working processes including any installed apps.) I could also imagine the browser not handling this well since it might well not expect a single process in the chain to be stopped (...or it might handle it by sending SIGCONT on seeing a process come back to waitpid() with WSTOPPED, defeating the layout modifier).
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