From all that I read, FF is all but slower on a Mac (renders webpages slower).
Here is the thing about fans. Just because they spin up does not mean your computer is over heating. They work off a step-up type of logic. I.E:
70 degrees or lower = Fan off or minimal speed
71-90 degrees = Fan at 20%
91-109 degrees = Fan at 50%
110 degrees and up = Fan at 100%
Your fan doesn't just kick on when your CPU or motherboard begins to fry.
I really don't understand why people worry when they are watching videos and the fan kicks on. If you don't like the sound, turn up the volume or use headphones.
The people that have said the FLASH is more resource intensive on a Mac are correct. But, it is working as it should. Until the developers tone down the FLASH resource requirements, know that your computer is working as it should.
Also, to those that would say "well it should work this way." Yes, maybe it should in your opinion. I'm just stating that it doesn't work that way now and you need to realize it.
EDIT: Yes, if you got a the newer Alu MB, you would see a far better improvement in playback of graphic intensive tasks. The Intel IGA is far less powerful then the dedicated GPU chip in the newer MBs.