I’m not very fond of applications that add themselves to the Windows startup because it means that the start of the operating system is slowed down as a consequence.
In my opinion, only a few applications have the right to be started at system start: included in the list are security programs as well as programs that you would start anyway after Windows is loaded.
I’m auto-starting Rivatuner for instance to keep the fan of my Nvidia graphics card from spinning too fast while I’m working on the desktop to reduce the noise they make.
Some users will undoubtedly like the idea of running a Firefox preloader that auto-starts with the system. This ensures that Firefox will startup faster when you click the icon. It is basically the same trick that Internet Explorer is doing by letting Windows start some of it’s needed files so that they do not have to be loaded when you click on the Internet Explorer logo.
A faster starting Firefox comes at a cost though. System startup will be delayed slightly because one additional application has to be loaded during that process. The Firefox preloader is also using some Megabytes of Ram all the time in addition to those used by Firefox if you start the browser.
If you use more than one installation of Firefox you have to make a decision which one to boost with the Firefox preloader. I guess it could be useful for some but I personally will not add it to the list of autostart programs.
To use the preloader simply install it on the system and keep the setting to run it at startup enabled. The application displays a system tray icon when it is running, which it is by default even after Firefox has been loaded, that you can use to configure it or reload or unload Firefox.
I don’t really think that it is necessary to keep it up after Firefox has been loaded but there is no automatic option to close it afterwards. You may want to consider terminating it manually though.