I know this is a really old extension, but it seems that sites with lots of Flash crap cause really high CPU usage in Firefox 2. I’m not sure if it’s just my installation but given that it’s a real pain in the backside I went hunted out Flashblock, which does exactly what you’d expect. Instead of loading a flash animation up, you just get a blank box with a little flash icon on instead. If you mouse over it, it gives you the option to play it.
Funny but Ars Technica looks pretty messy with Flashblock on!