The more cpu usage the more heat that is made, if you are overclocked or your system cant cool it self properly then you will over heat when the cpu is being used more.
My system is set to auto mode for the fans, when doing normal work my system is silent, when I then start to game and my video card and cpu are maxed out you can hear all the fans pick up in speed because the system is getting hotter.
When your cpu isnt being used much it under clocks, using less voltage and speed, thus making less heat. When your cpu is under use the voltage goes back up to normal and the cpu runs at the full clock it is suppose to.
So if you are over heating the question shouldnt be why is firefox doing it, as your system should be able to handle the cpu maxed out, the question should be why it isnt able to keep itself cool enough when under a full load. Check the fans, get the dust out so the air flow can be better and make sure your room temp is below 80F
Shane