Importing mail from Apple’s Mail.app isn’t built into to Thunderbird. That’s a major oversight I think, however the work-around isn’t that bad. There are a dozen steps or so, but really, it’s not hard.
This works for folders that are considered ‘On My Mac’ in Apple’s Mail.app aka ‘Local Folders’ or inboxes, outboxes, sent boxes whatever. Remember, that if you have IMAP mail, or your mail is all stored on a server somewhere, this is all not necessary. Set up Thunderbird and it’ll import all that stuff on the server. This tutorial imports email from local folders that are saved on your actual hard drive.
First things first, back up your mail! This is very important because loosing anything sucks. Second, get Thunderbird and get you account all set up.
All set up? Wonderful! Now there are two things to consider. Are you importing the inbox? Or a folder? Each way is similar, just slightly different. If importing the inbox, sent, or other main boxes, you won’t have to create folders. Just basically follow the instructions below. If you need help, let me know.
Lets import a folder. For the example my folder is called George
1) Create new local folder in Thunderbird called George
2) Quit Thunderbird
3) Go to Home -> Library -> Mail -> Mailboxes -> George
4) Right Click on the file named George.mbox and choose ‘Show Package Contents’
5) Find the file named just mbox, copy it to the desktop and re-name it George
6) Open a new finder window
7) Go To Home -> Library -> Thunderbird -> Profiles -> [your profile] -> Mail -> Local Folders
8) Find the file called George (no extension) and overwrite it with the George file from step #5
9) Start Thunderbrid.
When you click on the George folder all the emails will appear. They’ll all be marked read so you’ll have to sort though that. However, they’ll all have their attachments still there which is nice!
I think that covers it. Let me know if you have questions or issues.
Note: This worked fantastic for me on 10.3.9 and Thunderbird 1.0.x. Should work in 10.4.x also.