From the little I was able to find out, Preloader pre-loads various parts of your
Firefox browser into the memory (presumably the most important parts) so that when you want it to start, it is able to open much faster than normal. In fact when I clicked on the Fox logo to start the browser, it shot open like a politician being caught in a brothel with his pants down. The tabs fired open one by one as if they had drank five cups of coffee each. Firefox is now so fast it’s unnerving me.