As far as I understand, WebM/VP9 has been supported by both Firefox and Chrome for quite a while.
However, on my machine MediaRecorder.isTypeSupported('video/webm;codecs=vp9')
returns false
in Firefox, but true
in Chrome.
Software: Firefox 81.0, Chrome 85, MacOS 10.15. MacBook Pro i7 2016. Could it be hardware related?