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.