Import from Mail.app
From MozillaZine Knowledge Base
This article is out of date since tools -> import now supports importing mail from Mail.App. Until this article is updated please see https://support.mozillamessaging.com/en-US/kb/switching-thunderbird . Note that the "Switching from Apple Mail to Thunderbird" section is visible using Opera 11, but not Firefox 7, Commodo Dragon or Internet Explorer 9. So if you can't see it try a different browser.
Mail.app used to use mbox files to store messages like Thunderbird does. Starting with Mail.app 2.0 in Tiger Apple switched to storing messages in individual elmx files in order to make them searchable using Spotlight . Thunderbird does not know how to import the messages or address books from Mail.app so they have to be converted to a format it understands.
Create a Mbox file
There are several different ways to create mbox files that can be imported into Thunderbird. Your mail folders are normally stored under /Users/<username>/Library/MailMailboxes/. The inbox and sent folders for any POP accounts will be in a directory whose name starts with POP. Any downloaded IMAP messages will be in a directory whose name starts with Mac. The rest of the directories are any custom folders you created.
Convert the elmx files (recommended)
Use one of the following elmx to mbox converter programs:
Using File -> Save As
Mail.app doesn't have a command to export a folder but it still knows how to create mbox files.
This will create a mbox file. The main drawback is that Mail.app stores message status separately and it doesn't create any status headers when exporting, so all of the messages will be appear to be unread. If Mail.App added a file extension rename the file to have no file extension.
Use a script (least successful)
Each of the following scripts or AppleScript Studio applications supposedly can export the folders as mbox files but most users have problems getting them to work:
Import the Mbox files
Its recommended you use the ImportExportTools extension to import the mbox files. See "Install the ImportExportTool extension" in Importing folders for how to download and install it. If you get a cryptic error message its probably because you tried to download the extension by left clicking on the download link using Firefox, you need to right click and select "save link as" to avoid Firefox thinking its a Firefox extension.
If you import a mbox file and it doesn't appear to have any messages you can check that extension works by exporting a couple of existing messages (if you don't have any compose two messages and save them as drafts) and then import the mbox file it created. If that works then there probably was some problem creating the mbox file.
If you don't want to use the extension you can copy the mbox files to the local folders directory in the profile and then drag and drop the folders in Thunderbird to where you want them. Watch out for naming conflicts, that directory already has a inbox folder for example.
Import the address books
You can export your address books by dragging them to the desktop, creating a vCard file, and then uploading it to this web page which converts it to a .csv or .ldif file. Then import them using Tools -> Import -> Address Books -> Text Files. The MoreFunctionsForAddressBook add-on adds support for importing/exporting vCards. You could use it instead, but it might take longer to learn how to install and use it.
There is no support for importing or converting Mail.app's rule format to Thunderbird's message filters.
Older versions of Mail.app used mbox files to store the messages. You can rename them to have no file extension and then either move them into Thunderbird's local folders directory or import them using the ImportExportTools extension.
Mail.app caches all IMAP messages. If you have a IMAP account its usually easier to just create a IMAP account in Thunderbird to use the messages in the remote folders, either moving them to local folders in Thunderbird or configuring offline folders for them. However, if you want to migrate the messages from the Mail.app cache (to avoid a lot of network i/o for example) Emailchemy can convert Mac OS X 10.3's CachedMessages folders.