FF 52.1.1, Windows XP SP3.
Several months ago I followed the 1st post instructions, and all worked great. But today many youtube videos don't work ("If playback doesn't begin shortly, try restarting your device", and rotating endlessly). http://www.bbc.co.uk/html5 works in both modes. https://www.youtube.com/html5 display all blue checks. However, https://www.quirksmode.org/html5/tests/video.html does not display the H.264/MP4. Restarting FF or computer, cleaning cache and cookies don't help.
Some videos still work.
An example of working ones:
An example of not working ones (In Chrome all works OK):