I’ve had lots of troubles with Mozilla Firefox lately, videos in genral, and it got even worse on Youtube. After staring one video Firefox became unresponsive, sometimes it woke up again but most of the time I had to restart the whole program. I was sick and tired of this and I made endless searches for solutions.
Long story short. Downgrade from 34 to 33 and everything is smooth as a baby butt. Looking at the installation date I could see that the problems started around the time I upgraded Firefox. Happyness! 
Seems like something in FF34 make it sluggish, even without watching any video. FF33 feels less slugish?
I should mention that I also tried the reset function in Firefox and it might have had some or all the impact on the final result. Not sure. What’s for sure is that it took all my extension with it when it left!!! 
But it’s not more than a handful in my case. That’s what it does, though.