I tried to do an Import, but at the very least it seems extremely complicated. There is no “Thunderbird” option under Mail Import, though there is one for Netscape/Mozilla.
If you suggest Import, please provide detailed instructions. Otherwise, other suggestions would also be appreciated.
I would try to easy way – drag a folder in Thunderbird to the desktop. This should create a .mbox file. Then drag the .mbox file into mail. Make sure you drop the file where you want it. Repeat for each folder you have in Thunderbird – you should be able to select as many folders as you like at the same time.
This worked with every email client I’ve tried except Outlook.