It supports VP9, but not in WebRTC. Or at least, not without enabling a flag on each viewers computer[0]. I am running Nightly, and can verify that it is _still_ behind a flag.

I suspect they don't realize how many viewers this costs them, as this has been behind a flag for months already, meanwhile RTC users are being told "We recommend Chrome".

[0] https://stackoverflow.com/a/38701342/87520