Issue
I recently installed Ubuntu minimal on a new computer, and noticed that sometimes Youtube-videos didn’t work. All “old” videos seemed to work, but newly uploaded videos usually claimed that my browser did not support HTML5. Basically, I could have a video in one tab which did work flawlessly, and a video in another tab which claimed I did not have HTML5 support in my browser.
The error message was the something like the following:
Your browser does not currently recognize any of the video formats available.
Solution
It seems like videos on Youtube have different encodings, maybe depending on which server you hit, or what the video’s quality is (this is just speculation though). If you wait, you will usually hit a video later which has a “correct” encoding. So to solve it, you need to install support for more video codecs
sudo apt install ffmpeg
After that, restart firefox and it should start working