Flash has been dead in the browser for many years now. MPEG is not going to work out of the box on Linux. On Ubuntu, you need the package called ubuntu-restricted-extras which enables the browser plugin. For Fedora, you could try to [use dnf] instead of apt-get.

I'm using Firefox 76 packaged with fedora 30 but it wont display a video (weirdly youtube does work). I'm getting the message: "this content requires adobe flash or HTML5 video support..." I don't want to install flash because I've seen some content somewhere about security concerns.

It depends on how the video on Youtube was encoded and how new it was uploaded and such and you will likely not be able to play live streams on YT.

In order to fully support HTML5 players by pretty everywhere now (like Youtube, Twitch, news sites etc) for video, audio and also even games now you need to make sure you have at least FFmpeg installed as It provides codecs needed to fully support HTML5 players.

Patent and copyright restrictions are among reasons why many Linux distros do not come with packages such as FFmpeg in a new install without doing any updates after that includes packages like FFmpeg.

Adobe has made the Flash Player EOL at end of December 2020 so it is done for.

Thanks for the advice guys I found this: https://computingforgeeks.com/how-to-install-ffmpeg-on-fedora/ Following those instructions solved my problem on Fedora 30