Dear,DarkMT: I assume that other video formats are fine. So the problem is only with 1080 and 60ps.

• Check if all the squares are blue on https://www.youtube.com/html5
• Check what happens when you adjust manually the resolution to 1080 and 60ps.
• If firefox is slow when video is playing, try this addon: https://addons.mozilla.org/es/firefox/addon/h264ify/?src=api
• Try the safe mode, deleting cache and cookies, fresh firefox installation.

Hope this helps you . Regards!

I doesn't work but thanks anyways. The square that says "MSE & WebM VP9 " is unchecked. I've tried fresh install; it's just the new update. The video lags when i set it to 1080p manually. Also, the addon doesn't work.

Dear, DarkMT. Okay, try changing the next preferences on about:config : media.mediasource.enabled media.mediasource.webm.enabled Both should be on true. Succes?

media.mediasource.webm.enabled was in false. Now it's true. It works better but it still lags. Thanks !

New update came out and it works flawlessly. A lot of thanks for answering ! EDIT: It still lags with some videos