How to transfer my Yahoo Email acct. from Internet Exp. to Firefox, my new browser?

Firefox doesn't do email, it's a web browser. If you are using Firefox to access web mail, you need to seek support from your service provider or a forum for that service. If your problem is with Thunderbird, see this forum for support. [http://www.mozillamessaging.com/en-US/support/] or this one [http://forums.mozillazine.org/viewforum.php?f=39]