It's not often that I give a thumbs-up to a program that's as buggy as Thunderbird to Outlook Express 1.0, a.k.a. Tbird2oe (free). Why? The bugs aren't debilitating and as far as I know, it's the only program that will export your Thunderbird e-mail in its entirety to an appropriate folder structure. A program called IMAPSize also exports Thunderbird e-mail, but with that one you must process each .emf folder file individually.