I miss V2.0 of FF - since then it has become bloatware, and leaks memory like a sieve. And 50% CPU use on a 2+ GHz machine is just plain ridiculous
For me Firefox has always leaked memory like a sieve, even the earlier versions... It's still my primary browser but I honestly can't believe that they can't get the memory leaks under control... Even though my computers stay up 24/7 I have to regularly shut down and restart Firefox at least once a day or else it will drag the entire system down...
As for streaming videos, yeah there is something wonky with Firefox and it's not just Youtube I find the problem on many other sites as well... So now I always have a Chrome window open and God forbid even sometimes have an IE window open when it aggravates me... I might have to look at Opera again, I used to use Opera way back in the day when Netscape started to crash and burn way back around the post Netscape 3.0 days...
Edit also consider switching Firefox to the 'Beta' update channel vs the stable 'Release' channel... Personally I find their beta releases to be superior to the stable ones overall... Go figure...