With all extensions off Firefox gets the decoding error while Chrome 1440p works fine (7900 XTX). Adblock works on 1080p so this looks to be an HEVC issue, but Youtube 1440p/2160p still loads properly.
edit: uninstalling HEVC then going to Firefox about:support shows HW decoding did flip to Unsupported but the install extension link they provide goes to AV1 Video Extension instead, weird. Manually installed older Microsoft.HEVCVideoExtension_2.0.61931.0_neutral_~_8wekyb3d8bbwe and everything works again. For some reason the 2.4.45 or 2.5.10 do not with Firefox.
edit2: Windows appears to be force upgrading older versions via the Store, so it worked all of yesterday on the old and it's back to not working in Firefox with the new. Had to reuninstall and rereinstall to fix and disable store auto-updates in group policy.