Moving Mozilla Thunderbird emails to a new computer is easy.
Moving emails and all settings (including multiple email boxes) inside Mozilla Thunderbird from an old to a new PC is fairly painless.
Mozilla Thunderbird keeps all your email data and settings inside a Profile folder.
Its located here:
Just copy your Mozilla Thunderbird profile directory onto an external drive while the application is not active.
Install Mozilla Thunderbird on the new PC. Then close the application.
Copy the name of the profile directory created by default by Mozilla and paste it in Notepad.
Delete the default profile directory created by Mozilla.
Note: If you see two folders, then look inside both of them. The one with a lot of sub-folders is the one to delete and also the one whose folder name you need to copy.
Copy the profile directory from the external drive onto the place you deleted the profile directory.
Rename the profile directory that you copied to the name that you copied and pasted to Notepad.
Start Mozilla Thunderbird.
Your emails will all be there exactly like before with the same settings on the old computer. Even multiple accounts.