I use Debian's Firefox ESR, and for at least five major (ESR) versions there's been a bug where sometimes if multiple tabs try to do a web push notification at once, the entire browser process hangs maxing out a core and nothing I do will recover from that. (I use X11 with PulseAudio.) One of these days, I'll get around to reliably reproducing it. (I haven't tried `pulseaudio -k`, which might fix it if it's an issue playing the sound: videos hang kinda like that, though more recoverably, if the sound isn't working right.)
Same here, I also use Firefox ESR from Debian packages and I've observed Firefox to lock up hard a few times. Often to the point it affects the entire desktop so not quite sure if it's a bug in Firefox itself or possibly elsewhere e.g., Wayland.
I have not: I don't know how to do that. (I guess you can do it with a Python script?) The X11 utils reckons that the notification windows belong to the same client as Firefox, though, so I doubt this is the issue.