Most software, including Firefox, is designed to be run on a wide range of processors. This means that the software does not take advantage of the new features of the latest processors which could enable the software to be more efficient.
Some individuals and groups have built versions of Firefox using compiler options to optimize the code for specific processors. This has created a series of optimized builds that are noticably faster than the official releases.
One such site that includes optimized versions of Firefox 1.0 is Moox. This site also includes optimized versions of Thunderbird.
For discussions on optimized builds see the MozillaZine Third Party/Unofficial Builds forum
*** WARNING *** These are unofficial builds – If you have any doubts about using these builds then stick with the official releases from http://www.mozilla.org[/url%5D