I found this; https://stackoverflow.com/questions/29822573/html-video-returns-media-err-src-not-supported-error-after-playing-for-a-while HTML video returns MEDIA_ERR_SRC_NOT_SUPPORTED error after ...

and https://www.bing.com/search?q=MEDIA_ERR_SRC_NOT_SUPPORTED

zobrdrool said

I've installed W10 Enterprise N 1903 and find that online videos won't play

To actually answer the question. The issue is not due to Firefox but due to the N and KN Editions of Windows not coming with the appropriate Media Feature Pack from Microsoft.

The pack provides the codecs that is needed to fully use the HTML5 players used for video, audio and games. Many sites like Youtube, Twitch and news sites now days are using HTML5 players to be more cross platform compared to the more limited Flash Player Plugin from Adobe.

See https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

This page https://www.youtube.com/html5/ used to tell you what you supported for HTML5 players however it changed to saying you are supported if the browser fully supports.

You may think, but Chrome works. Well they kind of cheat by coming with its own codecs.

Thanks, James. I wondered if the Media Feature Pack might be a necessity. Looks like it is.