Firefox not playing videos on Ubuntu 20.04.4 LTS

Bro, a solution for your problem takes no more than one line :)

sudo apt install ffmpeg

That's all! After restarting your firefox browser the changes are applied and the browser plays any video content you wish.

P.S. I lost 2 annoying days searching the web for solution, trying different approaches, rolling back changes, reinstalling browser and so on, and try to imagine my face when this single line appeared the solution.

Just Open Terminal and paste:

sudo apt install libavcodec-extra

it works

You may also try

sudo apt install ubuntu-restricted-extras

See here for what it installs.