It was not an update, started happening to me on arch linux, where Firefox does not automatically update. Seems like some kind of timed enabling of https3 support.
Looks like HTTP/3 has been in Firefox for around a year
This is a great way to bypass continuity testing, I really pity all those people working the desks in hospitals right now using FireFox who are typically less savvy than your average HN'er in trying to get their work done.
If you ship a browser with a time-bomb you are utterly irresponsible.
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.
(1) I did not explicitly enable this
(2) The telemetry setting seems to have re-enabled itself on some update
(3) I don't want any services from Mozilla, I want a browser
(4) It worked until it blew up revealing that in fact, I suddenly did have service dependencies
And finally, the reason I use FireFox is exactly your last sentence, so to see that they are slipping this in under the radar is a pretty good reason to drop FF altogether, it looks as if they fail to understand the difference between shipping software and getting me hooked on some service that I am not even aware of existing. And on top of that re-enabling their telemetry when it was explicitly disabled. That really takes the cake.
Switching to a browser without telemetry and automatic updates won't protect you from this kind of network stack bug.
You argue that it wasn't an automatic update: I am pretty sure that the first install on this machine did not have HTTP3 support and that automatic updates pulled it in, end of story right there.
As for the telemetry issue: that's even worse because telemetry and all other forms of communication with the mothership other than automatic updates have been disabled on this machine, and has been silently re-enabled without my consent. That's a pretty gross violation of trust, and if that in turn causes me to lose a morning then that makes it even worse. Fortunately, today is not an interview day, but if this had happened two days ago the consequences would be terrible.
Finally, automatic updates would ideally just fix security issues and not introduce new, possibly unwanted functionality. I carefully select my tools for their purpose and I absolutely hate this brave new world where critical stuff suddenly stops working because some company could not be bothered to take their end users' interests a bit more serious.
Google and Cloudflare were not implicated here, it was FireFox that stopped working, Chrome still functioned just fine.
That's not how this works. HTTP/3 support is optional as far as I'm concerned and plenty of websites that I tested with do not support it and still failed due to this issue.
Absolutely unacceptable.
But mostly: I have no real idea what's going on, so I'm awaiting the postmortem.