Firefox's use of CPU resources has improved but hasn't gone away completely.
after I had been grappling with this issue for more than a year. First, on an Hewlett-Packard business ultraportable and then on the Apple MacBook Air. As I stated at the time, my theory is that many users don't notice Firefox CPU utilization on large, well-ventilated mainstream laptops. But it can be an issue on ultraportables, which are more sensitive to heat because of the obvious design constraints (typically under an inch thick).
On the HP ultraportable (model 2510p, running Windows Vista), CPU usage became a major concern. The cause was twofold. First, at least one HP 2510p SKU (using a 1.33GHz CPU--the configuration I owned) had a design problem. When the unit got hot, it would shut off without warning. That, combined with Firefox's CPU usage issues, as spelled out clearly by Mozilla in the link above, made for an unstable mix. Firefox, I determined after much trial and error, triggered most of the shutdowns on my HP laptop.
While this never happened running Firefox on the MacBook Air (which I use now), it did continue to cause overheating. I have both the original MacBook Air (which uses an older 1.8GHz Intel Core 2 Duo processor) and a newer version (which has a newer 1.86GHz Core 2 Duo processor). Running Firefox on the older Air became unsustainable at times because of the heat issue. While moving to Safari didn't completely alleviate the heat problem, Safari did not generate the level of heat and fan activity that Firefox did.
Before I go any further describing the issue, let me say this: that was then. Mozilla has made strides--at least for the Web pages I access most often--in improving Firefox CPU utilization on the Mac. In short, CPU utilization levels have come down since November of last year.
One way I monitor CPU usage is with a tool that anyone with a MacBook can access: the Activity Monitor. Using the newer MacBook Air and with the same 10 tabs open (including CNET's front door, a YouTube video, various Mozilla Web pages, and a couple-of multimedia intensive news sites), Safari and FireFox CPU utilization is now surprisingly even: jumping between 3 percent and 30 percent on both browsers, an improvement for FireFox over last November.
Separately, the Adobe Flash player, not surprisingly, also becomes active on Flash-based sites. CPU utilization for Flash typically levels out in the teens for the tabs (Web sites) I often use.
That said, Firefox still has a problem in two areas. One, it temporarily spikes at high CPU utilization levels and, two, it gets stuck at a utilization level well above Safari's when tabs are idle. The latter is the biggest difference with Safari and still the cause of heat and fan issues on the MacBook Air (particularly the older Air), though to a lesser degree than before.
Finally, in response to my queries, Mozilla said the "CPU usage improvement is the intended effect for shipping support for out-of-process plug-ins in Firefox 3.6.4." Out-of-process plug-ins, or OOPP, "provides crash protection for Windows and Linux users by isolating third-party plug-ins when they crash," according to a Mozilla Web page.
The problem for me (and Mac users) is that this technology does not yet apply to Macs, only Windows and Linux. So, I am seeing better utilization of CPU resources despite the lack of OOPP support on the Mac. Is the better CPU utilization my imagination or has Mozilla implemented other technology that I am not aware of? I would be interested to hear what readers have to say.
Notes: Two additional items to note briefly. There are other tools to monitor the CPU usage level, as the Mozilla support page states. Also, Firefox CPU utilization still gets stuck at very high levels on the older MacBook Air, which uses Intel integrated graphics, typically (with a few major news sites open) in the 30 percent to 40 percent range, while Safari settles down at around 10 percent.