Unless I am mistaken, Firefox seems to be using an awful lot of CPU power and I am not sure why, I have disabled any plug-ins I have installed to see if it makes a difference (although I am sure they are shown in a seperate thread anyway).
I don't feel like I have that many tabs open...or doing anything especially demanding with Firefox!
2.3Ghz i7 quad core processor
early 2011 macbook
What I am using in Firefox