CNET reports about high CPU usage problems with Firefox, which can cause overheating problems in some laptops, especially ultraportable models. You can read more about it over here.
Firefox has a CPU usage issue and, consequently, can cause overheating problems in some laptops, particularly ultraportables. That's what I've found over the last couple of years.
But don't take my word for it. This is documented on a Mozilla support page entitled "Firefox consumes a lot of CPU resources." The page states: "At times, Firefox may require significant CPU [central processing unit] resources in order to download, process, and display Web content." And forum postings like this one about a Dell Netbook are not uncommon: "Mini9 would get way too hot."