It definitely stopped in the middle of the night - I saw it happen in real time - what's more likely is that some third party is to "blame". Although the blame still rests squarely on the shoulders of Mozilla, since Chromium browsers all worked fine. Apparently the going explanation is that something involving HTTP3 runs into an infinite loop and never resolves (explaining why it also sends a core to 100%), and Cloudflare/Google/Some Other Thing updated to using HTTP3.