I can’t get Audacity to open some webm files. I get the error: “Audacity did not recognise the type of file '[full path of file] .webm. Try installing FFmpeg.”
The webm file contains an Opus audio stream and VP9 video.
I installed Audacity 2.4.2 and the 64bit FFmpeg libraries today. Audacity reports the FFmpeg library as: F(55.33.100),C(55.52.102),U52.66.100.
Unfortunately the Audacity developer that knows this stuff best is tied up with a big project for his day job until at least spring 2021. Unless another expert comes along I doubt it will be fixed until he’s back.