just realized this today as well. The video player is a complete disaster, even with forced h264 extension.
EDIT : this is really really weird. I'm pretty sure youtube played perfectly fine yesterday on firefox (mac os x here too). Call me paranoid, but could it be possible that they altered something to have the video stutter on firefox in the meantime ??
Makes my mac catch fire and still stutters.
edit: downvotes? you guys using some build I'm not? Because I'm on nightly.