Fixed this annoying problem with Firefox where flash videos would go all choppy/crackly depending on what I was doing. Anyway, with a quick Google two posts had the answer:
Sure enough, checking Task Manager's Always on Top option I could see my CPU was darting from the twenties to the seventies just while scrolling up and down.
Went to Adobe site, downloaded the uninstaller and version ten from their download centre/archive, and sorted. Crazy you have to go back three years to get a better performance. Anyway, thought I'd post it incase any other Firefox users are having this problem.