Importing Thunderbird folders into Mail.app 6
This is also posted on MacOSX Hints
With Mac OS X Tiger (10.4.8, to be exact), Mail.app 2 (2.1.1) and Thunderbird 1.5 (1.5.0.7) the import into Mail.app is very straightforward for importing the Local Folders and Inbox from a Thunderbird POP account. I did this as two imports, first the Local Folders, then the INBOX
Before doing anything, use Thunderbird to “Compact Folders” (which removes messages that have been marked as deleted in your folders).
1) in Mail, select File -> Import Mailboxes, then select Other 2) select the folder (HOME)/Library/Thunderbird/Profiles/Default.lay/Mail/Local Folder 3) Mail then finds all the importable mbox files therein, and all the nested folders and files as well. Confirm the import
Then repeat, but using (HOME)/Library/Thunderbird/Profiles/Default.lay/Mail/(account name) as the base for the import
This leaves you with two new top-level folders in Mail.app: Import and Import-2. You can then rename and re-arrange as needed for your purposes