I noticed after Chrome stopped working in Linux and I switched back to Firefox 71, that FF would not play a lot of videos or GIFs. This was true on some YouTube videos, all of Vimeo, and even locally opened MP4 (h264 with AAC audio, or h265, either HD or 4K) files I made. These files play fine on VLC. Apparently there is some kind of patent issue with MP4, and it would require that they pay something to the patent holder. Here is a link to the Mozilla page that alludes to this:
This may also relate to the problem I’ve seen with animated gifs, but I’m guessing.
It would have been nice for them to simply state that Firefox will not play MP4 files.
It isn’t related to plugins or adblockers. I dug around for a few hours reading various suggestions from years ago all the way up to a few months back. There were several saying “Oh, just clear your cache and cookies”. Bullshit! This has to do with FF not knowing what to do with a particular format.
I recompiled ffmpeg and a bunch of libraries, but that didn’t help Firefox. I hate that I have to guess at why the latest version of a modern browser can’t play common video types. If Mozilla would state exactly why they won’t play certain types of files on their knowledgebase, that would be helpful. Or tell us what steps to take to enable the browser to play those files. As I said, other programs on the same computer play them with no problems, that reduces the issue to Firefox.
I purged and reinstalled Firefox entirely using these steps:
sudo apt purge firefox
sudo rm -rf .mozilla
sudo rm -rf .macromedia [remove vintage Flash player stuff. May not exist.]
sudo rm -rf .adobe [remove vintage Flash player stuff. May not exist.]
Delete the mozilla folder
sudo rm -rf firefox
sudo rm -rf firefox
sudo rm -rf firefox-addons
Reboot your computer to get rid of any temporary files.
sudo apt install firefox
Firefox still is not able to play MP4 files. Ridiculous!