Well, I did verify that even the FF downloaded from mozilla requires ffmpeg in order to play the video when the embedded JW Player is served by the website.
I don't believe that this is an issue with required plugins at this point. As a new example, the video in https://www.iceni.com/transpdf.htm (screenshot https://pasteboard.co/JjDtMduG.png), does not load in my Firefox. However, using the developer tools in Firefox, I downloaded the embedded video (.mp4) from the link I saw in the Inspector. I was able to play this downloaded video in the default video player in Fedora with Gnome.
So I would appreciate if someone tells me if this means that this is not a codec issue, but maybe some permissions issue or something else thats happening with Firefox,
thank you!