No, all you have to do is install it. Firefox won't do anything to your Internet Explorer, although it may ask you if you want Firefox to be your default browse. You're free to say no. If you find that you don't like it, you can just go back to using Internet Explorer. I should mention that Firefox is capable of importing your settings and bookmarks/favorites from Internet Explorer, so you don't have to waste a lot of time customizing it.
It's better than that. They can run side by side if you like (I use Mozilla for everything, but if something refused to run, I copy the url over to IE and run it there).