It's concerning to me that it's even possible for Firefox to be "down". It's should be a web browser, not a service. Web browsers have uses beyond being connected to the internet. Does Firefox even work on a non-internet connected LAN?

This was extensively discussed yesterday: https://news.ycombinator.com/item?id=29918052

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Pieces of how this happened, and some of the fixes implemented, don't instill much confidence.

Amongst other issues, this was triggered by a lack of case-insensitive handling of the HTTP "Content-Length" header. https://phabricator.services.mozilla.com/D135871 has one of the commits that landed as a result of this, and while it does change the handling to be case insensitive, it raises even more questions on the HTTP/3 stack. For instance, this is doing a string search across the HTTP headers for the string "content-length". Does that string appear in a cookie? Well you just got the wrong content length. It's extremely concerning that this isn't downstream of something that has pre-parsed the headers and has them indexed by their parsed out names.

Firefox stopped working last night due to "Infinite loop in HTTP3 hangs socket thread". Seems to be working fine again.