I've tested the proposed patch below: https://github.com/amurzeau/debian-autobuild/releases/tag/firefox-esr%2F91.12.0esr-1%2Bnosse1_deb11u1
I've tested it on an Athlon XP 2600+ (Barton) with 3GB RAM. Everything seems fine. I can even play videos on youtube with smooth sound and if I play the videos at 144p then I basically get a slideshow, but that's fine for this machine :) Tested slither.io and I could 'play'. It was unplayable due to CPU bottleneck, but from software side it worked. I also tested WebRTC via https://networktest.twilio.com/ and it was working. Except the video-chat part, as firefox-esr doesn't have webcam support (based on the report at html5test.com). The only downside of this patch is that Pentium 4's with SSE2 will be somewhat slower, but at least it will work on all 32 bit CPU's, not just P4's. The alternative is adding the sse2-support package as a dependency for firefox-esr and use epiphany-browser instead of firefox-esr for i386 Debian installations by default. Thank you for the fix. I hope it gets merged soon.
