Firefox does not play videos (except for Youtube)

I have visited many sites offering solutions to this problem but nothing helps. I installed all the required codecs but the videos are not played.

For example, for for some videos there appears a message: "No compatible source was found for this media." or "Unfortunately, this video is missing or damaged and cannot be played.". Only YouTube videos are played fine. What is the issue and how to fix it?
The Ubuntu version is 20.04, and interestingly, Chrome has no such problems and can play all the videos.

Codecs installed:

sudo apt install libavcodec-extra
sudo apt install ubuntu-restricted-extras