Are you sure you actually noted a timed difference in loading times after adding the infamous /prefetch 1 switch?
Here is an article detailing various XP tweaks that are either not even recognised by the OS, or whose benefits are superficial at best. That tweak is mentioned.
I've noticed that IE loads alot faster after start-up after I've launched an Explorer session, usually by opening a folder. Since IE is integrated into the OS, it probably uses some of the same DLLs as Explorer, which get cached in RAM.
As Firefox is independent of the OS, it must load all its libraries and other data whenever it is launched after a fresh boot, since not of that will be cached.
Incidently, isn't there an option to automatically load Firefox into memory at startup?