yun

I've used the instructions in the start of this thread, and it worked for me, but now I have problems with some videos (like this https://www.youtube.com/watch?v=BUJiGnjGIoI) again, and https://www.youtube.com/html5 shows both H.264 and MSE & H.264 unsupported. I've rechecked the settings, the content of GMP-EME-ADOBE\17 in my profile, and my add-ons show Primetime CDM and Open264 Videocodec are always activated. SOLVED! It was media.mp4.enabled set to false

I don't think it'll be a problem, but if you know a better solution, tell us

Cool! It worked! I would have never come to such an idea myself Glory to Ukraine!

I started from this. And BTW I didn't update anything since the time when all worked,

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):