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.