From MozillaZine Knowledge Base

This article was originally written for Thunderbird but also applies to Mozilla Mail.

The best way to move your Thunderbird mailboxes to Mail.app on Mac OS X is "Eudora Mailbox Cleaner" - it will import all mailboxes (or a hierarchy of mailboxes like your complete ~/Library/Thunderbird/Profiles/youruserprofile/Mail/Local Folders/ folder) to Mail.app. The advantage of this over using Mail's own import feature is

• Message status flags (read, replied, ...) will be preserved, deleted message in mailboxes which have not been compacted in Thunderbird will not be imported
• Import of mailboxes with mixed or non-unix end-of-line characters will work correctly (Mail's import would lump several messages into on large message in this case)

You can download Eudora Mailbox Cleaner from its homepage. After mounting the disk image, you simply drop you mailbox (or mailbox hierarchy) onto the application's icon and wait for the conversion to finish. You will see your imported mailboxes in Mail after restarting Mail. If you are using Mac OS X 10.4.x, the imported mailboxes will initially appear empty - select "Mailbox → Rebuild" from the menu to force Mail to index the imported mailboxes and add the messages to the SQlite database storage it uses.

The formerly suggested manual method consists of the following steps:

• Navigate to ~/Library/Thunderbird/Profiles/youruserprofile/Mail/Local Folders/
• Identify mailboxes you seek to use in Mail.app.
• Each mailbox has three pieces to it - i.e. "inbox.sbd", "inbox.msf" and "inbox" with no extension.
• Take the file with no extension, hold down the option key (so it also stays in the folder) and drag it to the desktop.
• Rename with ".mbox" as file extension, for example "inbox" becomes "inbox.mbox".
• Open Mail.app and select "File Import", choose "Other" and navigate to the desktop and click "Choose".
• Your mailbox(es) should appear in the import window and will import into Mail.app.

If the above procedure doesn't work, see the Export mail into Outlook (Express) or Apple Mail article.

See also

• Importing and exporting your mail