I'm a bit scared of messing something up with code edits and I'm hoping there's a way to do a clean input. Googling around, it doesn't look like there is? I need to transfer everything from Thunderbird (PC) to Mail (Mac).
If you know your mail account settings, Mail should download all of the messages after you set up the account. What mail service do you use?
Have you even tried to download the emails in mail?