Firefox 38 youtube video stutter really bad!!!

[雄心]

As title this problem only appear in your Firefox 38, I've tried the newest official Nightly too but same problem.
I revert back to Firefox 37 and no more stutter.
What could be wrong here ?

[雄心]

https://www.youtube.com/watch?v=SXZ3qWAvbVs
This video for example, it keeps freezing any playing at the same spots even if I refresh, it was just at 4K at first, now even 1080p got the same problem.

[雄心]

I think it's only for 60FPS video, tried other 60FPS vids and all got this stutter problem.

[Tete]

Thank you for reporting the problem. On my system, it seems that the video can be played without any problem…

[雄心]

Can you play 4K smooth ?
I tried fiddling around and seems like the cause is vp9 codec, if I switch media.mediasource.webm.enabled to false then I can play 1080@60 smoothly because it's h264, but then 4K option is not available for me.
If you can play vp9 video at 60fps, please tell me your setting for webm and such …