I will try to provide as much information as I can. I have been having this problems for a long time now.
Basically, If I'm watching YouTube flash videos online, first couple videos will ran normally. After that... my computer will slow down... browser's CPU/Memory usage goes up. I believe after I close the first few videos, the resources are not being released by the graphics card. Thus this resource usage will pile up and then eventually my computer becomes very slow that I had to close my browser. I have to close my browsers for the graphics card to release those resources and then my computer will be normal. However, If I start to watch flash videos again, resource will keep piling up again and then I have to restart my browser again. Usually, after I watch maybe 3 or 4 videos... the memory/CPU gets pretty high and my computer becomes slow... then i have to restart my browser.
It is not only in Firefox, I tried with Internet Explorer too, it is the same thing.
I even reinstalled my Windows Vista and it would not fix the problem.
I did not have this problem on Windows XP. My other computer did not have this problem
My graphics card is Ati Mobility Radeon X1400.
I tried with the default Microsoft VGA driver, the problem is still there. I tried it with a driver from Dell, it is same thing...