Importing from Thunderbird to Mail.app

When I first started working with Mail I did an import of my Thunderbird stuff but today when I tried again, in order to recover deleted messages. I ran into a lot of problems and the help I found didn't work. All of the things it made sense to import were grayed out.
The solution is to import the folder called Mail.
This may not work for me though because currently I only see Sent, Trash and Unsent. Perhaps Mail has something that records what you have previously imported.