The reason it can take longer to start up is cos Windows automatically loads IE, (which makes your whole system run slower), whereas mozilla isnt autoloaded...

And im surprised you say IE loads pages faster, I have always found firefox renders them much quicker than IE

But as for me, Im running AMD 64 3200+ and 1GB of RAM, so the split second that it takes firefox to load doesnt seem any different from IE, and Firefox has tabbed browsing which I use and abuse (usually like 30-50 tabs open)