The following answer to another question may help you. The answer is written by the user "accolade".
XUL Profiler is an awesome extension that can point out extensions and client side JS gone bananas CPU-wise. It does not work on a per-tab basis, but per-script (or so). You can normally relate those .js scripts to your tabs or extensions by hand.
It is also worth mentioning that Google Chrome has built-in a really good task manager that gives memory and CPU usage per tab, extension and plugin.
Let me add some more info to accolade's answer. As of January 2012, the latest version of XUL Profiler is 1.0.4, released December 2008. It's only certified compatible with Firefox 2.0 - 3.6.*. So it clearly needs a new maintainer.
I wonder if there's any way to force the extension to work on newer Firefox versions.
Also, I wonder if it works well to downgrade Firefox temporarily in order to use XUL Profiler with your existing tabs on a certified-compatible Firefox version.