I needed to convert an .mp4 file to a VP9 .webm file. FFMpeg defaults to using VP8 for .webm files, but VP9 is broadly supported by both Firefox and Chrome now so I'd rather use that.

Uses the default, and makes a VP8 files:

ffmpeg -y -i input.mp4 /tmp/output.webm

Force the codec to be VP9:

ffmpeg -y -i input.mp4 -codec:video vp9 /tmp/output.webm