I am sure most Firefox power-users have seen the optimized builds for the various operating systems. It is also possible to make changes that will increase performance yourself using the about:config panel.

Basically just run Firefox, type about:config in the address bar and use this page as a guide for what you can change for your specific situation (fast computer/slow connection, slow computer/fast connection, etc.).

For an additional reference, check out the Firefox Tweak Guide.