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I have tried AV1 video on Chrome and it seems to be using the same amount of CPU as VP9 on my computer.

I can't test Firefox support for AV1 on my Mac, because it seems to work only on Windows.

It's only on windows by default, but you can enable it using about:config flag media.av1.enabled

I've been running with AV1 enabled on Linux (via about:config) for a while - maybe flipping the pref will work for you?