This isn't a time bomb, it's a symptom of the way modern browsers tend to have centralized I/O which means disk and network traffic goes through a single chokepoint. HTTP3 traffic appears to be able to cause one of Firefox's socket threads to hang, and since everything goes through it, all your network traffic is now dead. Chromium uses a similar model of routing all I/O through specific places, so it's vulnerable to similar sorts of problems (though AFAIK it has never failed this badly in production).
Basically, this could have happened at any point if a production service run by someone like Google, Facebook, Cloudflare etc managed to trigger a sufficiently bad bug in a modern browser's network stack.
If not for the centralized I/O this would just hang a single browser process, which isn't as impactful since Firefox and Chrome both split content out into many processes. It would also make it more obvious which server(s) are responsible since only certain tabs would be dying. FWIW, as far as I know this centralized I/O model was popularized by Google, not Mozilla.
