With either IE or firefox, a runaway script can eat 100% cpu time on that CPU core. But I often have 15+ browser windows up at a time. I want to identify which browser window is out of control so I can close it, but going to the task manager only identifies that the program (IE or firefox) is using the CPU, not which window.
I often have the same problem. While this isn’t an answer to your question, you may find the Firefox add-on “Session Manager” to be helpful, since it lets you save all the tabs and windows you have open and re-open them as desired.