You can easily convert Mac Maildir files using MacUncle Maildir to Thunderbird Converter. The tool supports Mac OS Machines.
Start MacUncle Maildir to Thunderbird Converter on your Mac Computer.
Select Maildir file using Add File(s) or Add Folder(s).
Choose saving option as Thunderbird.
You can also Choose Destination path if required.
Mac Maildir to Thunderbird conversion for Mac OS process completed successfully, you can see the message of completion.