First Download and Install the tool to Import Apple Mail to Mozilla Thunderbird. Step 1: Start the tool on any macOS above 10.8 Version. Step 2: Select the option Convert Email or Convert Contacts & Calendars. Step 3: Now either choose Convert configured Apple Mail mailboxes data or Let me select my AppleMail Mailbox data. Step 4: Upload AppleMail files using Add File(s) or Add Folder(s). Step 5: Further, choose the Thunderbird and location. Step 6: And press the Export button to start the migration. After successful completion of the transfer from Apple Mail to Thunderbird on Mac OS, a few messages are displayed.