The FAQ entry (http://www.mozilla.org/support/thunderbird/faq#mail.app) says:
How do I import e-mail messages from mail.app (MacOS X)?
Go to ~/Library/Mail/ and root out the mbox files in the finder. These are actually OSX packages.
No they are not packages!! First of all, to get to the .mbox files, they are found in
/Users/<username>/Library/Mail/Mailboxes/
and here, packages are not found, rather folders like John_Doe.mbox within which in all cases is found another folder called Messages. Inside Messages is found the actual messages with numeric names like 642433.emlx and a Info.plist file at the same level. And, there are no Inbox entries in the Mailboxes folder. The INBOX.mbox is found in the folder called "POP-[name of user email account]. Again, the actual messages are in a Messages folder with numerically named messages and a .plist file.
Select Show Package Contents and a new window will open revealing several files including a file named mbox. Copy that file to the subfolder called Local Folders in the folder Mail in your profile folder (On Mac OS X, the path is usually ~/Library/Application Support/Thunderbird/Profiles/xxxxxxxx.default/) and rename it to something meaningful. This process needs to be done for each of your mailboxes. Finally start Thunderbird to see the new mail folders.
This is totally different from the Mail directory for Thunderbird, which is absolutely NOT in Application support, at least not on my machine. I found the Mail directory for Thunderbird at: /Users/<username>/Library/Thunderbird/Profiles/2mc8t4zq.default/Mail/Local\ Folders/ (in the Finder of course the last expression is 'Local Folders' - the backslash represents a space in Unix parlance) - anyway, this is a totally different location than given in the instructions.
Okay, I find different info. in knowledge-base, http://kb.mozillazine.org/Profile_folder#Thunderbird, where it lists 3 possibilities:
~/Library/Thunderbird/Profiles/<Profile name>/
~/Library/Application Support/Thunderbird/Profiles/<Profile name>/
Users/<user name>/Library/Thunderbird/Profiles/<Profile name>/
If one followed the FAQ first, he/she wouldn't have known this.
And, the files are altogether different. Whereas Apple files end with .emlx, Thunderbird is organized totally differently, e.g. a file called john_doe, then john_doe.msf, then a folder called john_doe.sbd, and this folder is empty! Same with theothers, e.g the inbox folder. Where are the real messages? What extension do they have? The file (e.g. john_doe) without any extender seems to have the biggest byte count. I guess they are all in there, as opposed to individually organized with a .plist as in Apple's Mail. With these differences, I can't see how it is possible to import the Apple files to Thunderbird.
I'm prepared to read up on how this program works, 'cause it has some features that I like that are hard to activate in Mail, like return receipts, and if it works, encryption and hopefully GNU Privacy Guard. First there must be a clearer set of instructions for importing mail.app messages in Tiger. The stuff I have read is sometimes either very sparse when it comes to Mac users, or one has to try to extrapolate from instructions for the Windows version. If I master this import trick, I'll certainly be willing to write up a Mac - Tiger specific how-to, but I need help myself to get there.
Last minute note: just before posting this I found a Sept. 27 2005 message that says
I guess that is what I needed to know. Or has the situation changed? I don't know where the $25 program is, poster didn't say. I respectfully suggest the FAQ be updated to warn users of Tiger's version of Mail.app about this. I think Thunderbird is so powerful and sophisticated compared to Mail that I can only hope that a fix for Mac users, or if there is one, that it be identified. I apologize if any of the above sounded like a rant, more frustration around a glitch in what otherwise is probably the most important e-mail and news app ever devised. For Mac users reading this, ignore macosxhints info on the subject, it is old and now incorrect like the FAQ.Similarly, the kb page entitled "Importing and exporting your mail - MozillaZine Knowledge Base" is out-of-date and doesn't cover Tiger version of Mail.app. If someone knows differently, even where that commercial prg. is located, I'd sure like to know!Mail.app recently changed thier mail format making it difficult. There is a program that can help but unfortunately, it costs $25. I'm trying to find an alternative free program. When they [Apple] make changes, it takes time for other programs to catch up.