Firefox Preloader is an open source, windows only application that reduces significantly the time it takes to start your Firefox browser. It does so by preloading some components of Firefox, thus reducing the start-up time just to about two seconds.
Naturally it bites a bit of your Ram, but to keep Firefox always ready for start-up, about 30MB of Ram would always be allocated to it, even when the browser is not running.
What else; If Firefox is your most used application, then you add it to your start-up programs.
Firefox Preloader is compatible with Firefox 2 -3.