Hello, did you install the media codecs during the install?

Mkll said

Hello, did you install the media codecs during the install?

The codecs from ubuntu? I've installed the restricted extras. I don't know what are the codecs that I'm missing.

Very weird, does the issue happen in Safe Mode? Navigate to about:support and click on Start Firefox in Safe Mode, and try to reproduce the issue there.

Mkll said

Very weird, does the issue happen in Safe Mode? Navigate to about:support and click on Start Firefox in Safe Mode, and try to reproduce the issue there.

The same happens in safe mode.

You can see what your system supports for HTML5 player at https://www.youtube.com/html5 as all checks should be blue.

Firefox on Linux has not used Gstreamer for a long while now as FFmpeg is used.

Seems that H.264 is not working..

Not sure what specific packages you need fort your Linux distro but you may not have FFmpeg package(s) installed in your package manager.

If Firefox was still running when you install the packages then you may need to restart Firefox before checking the https://www.youtube.com/html5 test page again.

Keep in mind that twitter videos is not perfect as even when you do fully support HTML5 player, a video in a tweet can sometimes show a error or such.