You could try the browser Camino, also from Mozilla. In my view, it does nearly as good a job as FFox, but uses fewer system resources.
Flash video developers and web page designers have charged ahead in leaps and bounds with little regard for the hardware required to run what they put on their web pages. When you encounter this sort of problem, it may help to quit absolutely everything else that might be open (but your screenshot suggests you'd done that already).
I see too that you have 1 Gb RAM. Usually sufficient, but I'd recommend upgrading that to its maximum of 2 Gb. That won't affect the CPU directly, but will help 'smooth' the processing.