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 ?
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.
I think it's only for 60FPS video, tried other 60FPS vids and all got this stutter problem.
Thank you for reporting the problem. On my system, it seems that the video can be played without any problem…
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 …