Firefox does not play YouTube HTML5 videos for me. There seems to be no way for me to make them start. They to not autostart and the lower left button (the triangle) does not start the video. The video seems to download alright.
I have tried disabling extensions, starting withfirefox -P and selecting a fresh profile. I have edited with about:config so that https://www.youtube.com/html5 displays HTMLVideoElement, H.264, WebM VP8, Media Source Extension, MSE & H.264 and MSE & WebM VP9 as all enabled and the page show that the HTML5 play is used when possible.
Epiphany neither works. Previous Flash-based playback worked.
This was due to JACK (/usr/bin/qjackctl) being started. Pressing 'Stop' in the JACK Audio Connection Kit started the video.