Firefox plugin CPU usage

You could analyze the CPU usage using Process Explorer. Right-click on the Firefox process and select properties. On the Threads tab you will see the different threads including add-ins such as Flash or Acrobat with their CPU usage listed.

EDIT: In fact, it should be possible to monitor threads with perfmon, too: Right-click to select Add Counters... and then choose Threads as performance object.

I'd guess your best option would be to test your plugin in a seperate Firefox process, but you're probably doing that anyway.

For real profiling you should use Firebug. I'm not sure about it, but I think it is possible to run XUL apps inside of Firefox (without integrating it as a plugin). If this is not an option then you could maybe separate out code that you suspect to be slow into a web page and profile it with Firebug. This would of course only work for stuff that is not interacting with the Mozilla core.

Actually Firefox does have a built-in "Task-Manager" for a few years now. Just type about:performance in the URL. It shows Name, Type, Energy Impact and Memory of each tab and add-on.

If you want to dig deeper Shift + F5 opens the performance tool where you can record e. g. opening a website and look into timings etc.