Manual Migration: Importing Mail the Old-Fashioned Way

Thunderbird also allows you to import mail, address books, and preference settings from other email clients. To accomplish this task, navigate to Tools | Options. The Import Wizard should launch and present a series of radio buttons asking you what you want to import (Address Books, Mail, or Settings). Follow the wizard, and your mail, address books, or preferences should be imported from the directory you specified into Thunderbird.