[def-]

I get VP9 videos in Firefox apparently, probably depends on the video and your youtube settings:

Running MOZ_LOG="PlatformDecoderModule:5" firefox shows that VP9 works:

[RDD 15027: MediaSupervisor #1]: D/PlatformDecoderModule AppleVTDecoder[102edcfa0] Creating AppleVTDecoder for 3840x1920 VP9 video [Child 15023: MediaPDecoder #1]: D/PlatformDecoderModule AudioTrimmer[11fec72e0] ::PrepareTrimmers: sample[0,21000] no trimming information [RDD 15027: MediaPDecoder #2]: D/PlatformDecoderModule OpusDataDecoder[102ef4040] ::Decode: Opus decoder skipping 312 of 960 frames [Child 15023: MediaPDecoder #2]: D/PlatformDecoderModule AudioTrimmer[11fec72e0] ::HandleDecodedResult: sample[0,21000] (decoded[0,13500] no trimming needed [RDD 15027: MediaPDecoder #1]: D/PlatformDecoderModule AppleVTDecoder[102edcfa0] AppleVTDecoder: using hardware accelerated decoding

AV1 hw decoding on M1 Pro indeed is not supported even in Firefox.