Attempting to play a udemy.com video and message appears in playback area, " Video error
We tried several times to play your video but there was an unforeseen error. We have notified our engineers. Visit https://helpx.adobe.com/flash-player.html to check that Flash is enabled on your browser."
The Adobe Flash Player is installed via software,
Flash player is enabled in FireFox extensions. Should I see it in plugins?
I think I have what I need, but I am obviously missing something.
I had to Install google chrome to watch Udemy videos
You do know that is the wrong answer? 
It works on Fedora and Ubuntu …
You do know that I am, at least partly, teasing you.
I will go through the steps and let you know how it went for me. Thank you.
So how did it go?) Can you follow your courses?
Same thing happened with me.
I’ve started a course from https://courses.edx.org, CS50’s Web Programming with Python and JavaScript.
My normal operating system is EndeavourOS ( Arch based), now trying out Clear Linux as this is used by couple of my friends.
Trouble is that the videos from edx do not play in Firefox. I get the error " No playable video sources found."
On checking the forum, I came across Q&A link.
The steps are fairly simple and I am able to see the video from edx now. Somehow feel that before starting the course I’m getting a feel of whats in-store for me in the course 
Its strange that a developer specific distribution is not able to play educational videos while a generic distribution like EndeavourOS has no problems.
… one would think that a professional would know about various problems with various licenses under linux (why x can do what y cannot do). my personal opinion, independent of intel/cl/devs and your codec problem with ff: this edx-site is also completely unacceptable in other areas, and an example that the web is beyond repair (my early days: html/css only, except the js-zombie & of course no tracker & co. at all). sometimes i wish gödel was right.