"Task Manager" addon for Firefox?

With Firefox you can type about:memory and it will show you a breakdown of memory usage (might be available on older versions too). I don't know how useful this breakdown is for you. I too would like to see how much each tab and plugin has in use as far as memory and CPU goes.

StatusbarEx is a Firefox extension that will show system information on the statusbar of Firefox, such as the memory usage of system & Firefox itself, network speed, system power status, etc.

However, it doesn't work 'tab specific'. since Firefox (unlike other web browsers) is using threads instead of processes to open tabs. I doubt this is possible at all.

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.

[XUL Profiler] is a Javascript profiler. It shows elapsed time in each method as a graph, as well as browser canvas zones redraws to help track down consuming CPU chunks of code.

Traces all JS calls and paint events in XUL and pages context. Builds an animation showing dynamically the canvas zones being redrawn.