Which is it? Can't download, can't load the page, can't play the clip . . . . . . .

Will not download.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for a download manager. Many websites, like Youtube, don't want the clips downloaded, just played.

Cappelen said

Will not download.

Download means you are saving say a video file to disk, not viewing it in a video player in web browser.

Youtube uses HTML5 player when possible and falls back to Flash Player if needs be.

If you have Windows 10 N or KN version then that could be the reason you cannot view HTML5 video?. To confirm everything on https://www.youtube.com/html5 should be checked if you can view HTML5 video.

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

The problem was an extensions "No Script". Thanks to all for your support.

That was very good work. Well done.