Firefox won't play MP4
After a while resting it, I decided to try again. First post says again to install ffmpeg package. Instead of trying installing it by apt, I tried ffmpeg -version. This showed me an error that libva.so.1 was not found. libva.so.2 was available. Making a symbolic link named libva.so.1 (to libva.so) and doing this for couple other libraries worked for me.
For ubuntu 20.04, run:
apt install libavcodec58