Why do I have to enter my password to get to yahoo mail, but not on the first load?

[bizlar2002]

When I open firefox, it will go right into my mail and will stay there as long as I keep it open in a tab. If I navigate away from mail, and then try to come back to it, it will ask for my password. However, I can simply close firefox and reopen it and it will go back into my mail like nothing ever happened.