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?
After looking into Firefox code, I found out that as of today FireFox MediaRecorder does not support WebM/VP9. The source code I looked at is here MediaRecorder.cpp
The implications? 60 second video recorded with MediaRecorder on Chrome 5-10Mb (WebM/VP9), on Firefox 40-50Mb (WebM/VP8).