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.