How to determine web page cpu usage?

When I have a lot of web pages open in Firefox, sometimes the Firefox cpu usage can grow to 95% or more, making all of the pages slow. I've noticed that closing pages on certain sites can significantly lower the cpu usage but it is just a trial and error process, and I don't really want to close the pages if they aren't using significant cpu.

Is there some way to determine which pages (or add-ons) are using the most cpu, so that I can concentrate on closing those? Or some way to stop pages that are not even on visible tabs from using the cpu at all?

I've looked for a Firefox add-on that could report the information but haven't found any, and some add-ons I've tried such as Firebug actually seem to make the cpu usage far worse.

I am using Firefox on Mac OS X.

You could use Chrome for this. It has the ability to tell you how much memory is consumed by each tab individually. Chrome for Mac.

Firefox 4.x should add multiprocess support, which should solve your problem