Testing a Pre-Release, Parallel Firefox
Firefox, in its official version, still lacks support for multi-threading (running on different processors), though Chrome and Internet Explorer 8 both have this feature. A Firefox project called Electrolysis is underway to close this gap. A blog author tested a pre-release version of Firefox that loads different tabs in parallel, and he chronicles his findings, including a huge speedup in Javascript vs. Firefox version 3.5 (though the pre-release still lags Chrome in many of the tests).
Firefox, in its official version, still lacks support for multi-threading
Firefox certainly supports multi-threading. A thread is not the same thing as a process.
You do realize that your Prescott Pentium IV is more power hungry than Intel's current faster offerings, right? Perhaps you should buy an AMD if you despise intel and would like to be greener.