Any Youtube video I play in Firefox runs at approx 5 times normal speed.
Also try running the video here at the BBC, this is jerky, showing quick staccato movement interspersed by short freezes.http://www.bbc.co.uk/weather/
And no sound from either.AHA!! Sound was the problem
. I played Banshee and found no sound there either. So I fiddled with sound in control centre and when I chose a different output device the sound appeared. This then made all the videos play properly.
I was going to delete this post but then I decided to leave it here so others with the same issue can check this out.