.avif files should only be requested if your browser thinks it supports .avif, otherwise .jpgs are requested, thanks to the <picture> element being used. I got the .jpg versions in Firefox (91).
CanIUse [0] says the format is supported in Firefox only "via the image.avif.enabled pref in about:config." Sounds like you have it enabled but either actual support is buggy or the site's .avif files are somehow out of spec.
I'm not sure if this is an image error or a Firefox issue, I have heard about Chrome being more lenient than spec when loading avif images. I might look into that a little more and maybe file a Firefox bug to see what they think, I'll update here if so.
This does appear to be a case of there being an actual spec issue with the image, and Firefox being more strict than Chrome. If anyone is curious I can post the exact errors (although they're rather verbose), otherwise I ran it through the AOM validator[0].
I'm finding two relevant Firefox bug about being more strict than Chrome[1][2], but neither of them seemed to cover this case specifically. I'm considering filing a bug about it, but doing so would link my bugzilla account (full name and whatnot) with my HN account.
You're running not-even-beta software so it not working is the most likely explanation.
The .avif files do load in Chrome. I found them to be noticeably blurrier than the .jpg so I think they overdid it on the compress. It makes the site look more like Amazon Prime Video than Netflix.
CanIUse [0] says the format is supported in Firefox only "via the image.avif.enabled pref in about:config." Sounds like you have it enabled but either actual support is buggy or the site's .avif files are somehow out of spec.
[0] https://caniuse.com/avif