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.
With Tbird2oe all you do (after switching the program to English from its native Czech) is point the program to the Thunderbird e-mail folder (most likely something along the lines of C:\Documents and Settings\User name\Application Data\Thunderbird\Profiles\random name\Mail), select an output location, then press start. The program processes all your Thunderbird e-mail and places the resulting individual .eml files in appropriately named folders, albeit very slowly--it took about two hours for my 27,000 e-mails.
But you wanted to read about the bugs. When I first tried to browse for the input and output locations, only the Program Files folder was listed: I had to reselect the drive from a dropdown menu to see all the existing folders. Also, the browse feature only worked once per session--I had to restart the program to change the input or output location whenever I made a mistake.
Insects or no, this program (in conjunction with Outlook Express Eml Importer, better known as OEImporteml.exe) saved me a heap of work getting my e-mail from Thunderbird to Outlook. Did I just say Outlook? Yup, OE was only a way-stop. Outlook won't import Thunderbird files, however, it will import e-mail from Outlook Express. So if you need to migrate e-mail from Thunderbird to Outlook Express or Outlook, get out your flyswatter and download Tbird2OE.