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."
The Mozilla support page goes on to say that "you can review and monitor CPU usage through specific tools" and describes ways to limit CPU usage, such as: "A Firefox add-on, called Flashblock, allows you to selectively enable and disable Flash content on Web sites."
Let me describe my experience. I find that tab for tab, Firefox uses decidedly more resources than other browsers--Safari, for example. And in the past (when I was actively using a Windows Vista-based machine) Firefox also compared unfavorably with Microsoft's Internet Explorer for CPU usage.
More specifically, here's the behavior as I see it. When I'm accessing sites with multimedia content such as the CNET front door, Firefox CPU usage will bounce around between 30 and 60 percent, and sometimes spike higher (80 percent and above), as indicated by the Mac OS 10.6.2 Activity Monitor.
On the other hand, the Safari CPU usage with the same pages open is much lower--typically between 2 percent and 10 percent.
My theory is that most users don't notice this because in mainstream laptops, this isn't an issue. But it can become an issue in ultraportables--typically under an inch thick--which are more sensitive to heat because of the design constraints. The ultrathin Apple MacBook Air, which I use as my main machine, is a good example.
The fan is usually an audible indicator of CPU usage issues. When I'm using Firefox and I have tabs open on multimedia-rich sites (which is par for the course these days), the Air's fan will almost invariably kick on and stay on until I close the tabs. As I write this, the fan has finally shut down after I closed the Firefox tabs (e.g, CNET front door). Those same tabs in Safari are still open and not causing any significant spike in CPU usage or fan activity.
When I contacted Mozilla, a technical support person guessed that Safari is possibly better at optimizing Flash-based sites compared to Firefox. And that may be true. However, I had similar issues before when I was using a Hewlett-Packard business ultraportable (also very thin like the Air) that were not necessarily tied to Flash usage. In short, Firefox was less efficient with CPU usage compared to Microsoft's IE 8. And the behavior was similar. The HP laptop would quickly heat up and the fan would kick on.
Finally, let me reemphasize that I'm guessing that most users don't notice this because heat dissipation is not a big issue for mainstream laptops that are not necessarily thermally-challenged when accessing multimedia-rich Web pages. That said, this has been a steady problem for me because I use ultraportables almost exclusively and has forced me to limit my use of Firefox.