From the author’s website (quoting):

Firefox Preloader is a utility that is designed to load parts of Mozilla Firefox into memory before it is used to improve the its startup time. (end of quote)

• “Firefox Preloader” on sourceforge.net:
  sourceforge.net/projects/ffpreloader

[Update from 2007-05-26]
I decided not to use the preloader any longer for now because it has some problems with properly shutting down Firefox when XP is shut down, see the bug tracker list on the project page. I am now using the MinimizeToTray addon for Firefox (also available for Thunderbird), which keeps the program in the system tray when being closed, so that it is actually always open in the background and does not need to be restarted each time.