With Firefox 67 on a quad core Mac I'm getting solid video playback on AV1 on Youtube 1080p
If you want to test this right now, here is how to enable AV1 in Youtube and test videos.
Goto the youtube testtube page and click "Always Prefer AV1" https://www.youtube.com/testtube
AV1 Beta Launch Playlist
https://www.youtube.com/playlist?list=PLyqf6gJt7KuHBmeVzZteZ...
And click "stats for nerds", if it's working it should show "av01" under the Codecs tab.