You could try disabling Flash plugin's protected mode. The protected mode feature of the Flash player plugin has security benefits, but seems to have compatibility issues on some systems. There is a new hidden setting for this: (A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful. (B) In the search box above the list, type or paste flash and pause while the list is filtered (C) Double-click the dom.ipc.plugins.flash.disable-protected-mode preference to switch its value from false to true. This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.

1) Enter the config by typing about:config in the address bar. 2) Enter media.mediasource.webm.enabled and press enter. 3) Right click on it and change to true, for some reason it is set as "false" which makes MSE & WebM VP9 not work properly. 4) Visit: https://www.youtube.com/html5 and check that everything is OK. 5) Try to watch a video. It should work now.

Thank you very much for your kind and intelligent response to my problem. When I tried it, I got a message from Microsoft saying there was a problem. They apologized. I sent the error report but the issue remains unsolved.

I think that is a robot response. Did it show you an error report?