Since Firefox is a web browser and not an email client (which is what I'd use to access my email) I assume that you're trying to access a web page that lets you log in to your email. Further, I'm assuming that you used to have that page set as your home page but that, somehow, MSN has replaced it.
If that's correct, you simply need to reset your home page. Go to the log-in page for your email. With that displayed on your screen, go to Tools > Options > General. Under 'Startup', click 'Use current pages', and then 'OK'.
Chris