Can't import Thunderbird mailboxes to Mac Mail

I thought I'd take another stab at using the Mac mail client and once again was stymied by the fact that it crashes while trying to import my Thunderbird mailboxes. What's up with this?!

I've found an app addressing this problem that someone wrote originally for Eudora mailboxes but works for Thunderbird mailboxes too (Eudora Mailbox Cleaner). Do I really need to do something like this?

Whine

"Thinking is hard."