Why does firefox cpu useage drop when I open task manager?

Firefox can be running at around 99% cpu, typically on a facebook page. I can tell because the cpu fan will go to high speed and I can barely type anything. I notice that, if, when this happens, if I open Task Manager, almost immediately, the Firefox cpu utilization will drop dramatically from 99 % to the 30's. Initially I thought this was a fluke but no, there is a definite link between having the task manager open and looking at it (not minimized) AND the cpu utilization by firefox. My guess is, firefox has code that watches for the state of the task manager and it if is open and not minimized, whatever firefox is doing that is cpu intensive is curtailed