Both macOSXHints and mozillaZine carry the same tip for importing email messages into Mail.app from Thunderbird. It involves copying the relevant mailboxes to the Desktop, adding the “.mbox” extension to the files and then importing them via Apple Mail’s Import feature.

Contacts can be exported from Thunderbird’s Address Book in either CSV (Comma Separated Value) or tab delimited format.

You can then import them into OS X’s Address Book using its native import feature or using a third-party app like Address Book Importer (which is free for a “one-off” use).

UPDATE: In the comments to this entry, Andreas Amann points out some problems with the tip on masOSXHints and MozillaZine. He notes that Eudora Mailbox Cleaner now handles the importing of Thunderbird mailboxes as well (and better). Thanks.

Going the other way? (Shame on you!) CosmicSoft’s emlx to mbox converter looks promising for Mail 2.0 users on the way out. Use it, then follow the second half of the instructions for importing emails from kmail in the Thunderbird FAQ.