“MOOX”:http://www.moox.ws/tech/mozilla/ is one of many sites offering optimized Firefox and Thunderbird binaries for download. Among these, MOOX is the most complete I have seen, with binary compiled daily from BRANCH and TRUNK, and each of the releases. The site discussed the optimization to some detail. From what I can gathered, these are mainly two things: code optimization with different options during compilation (and using different compiler) and processor optimization to match the instruction set and architecture.
I’m on a Pentium III machine and am running the M2 version of Firefox 1.0 RC1. *It is noticably faster.*