Mail Backup X is a great addition to saving all your emails on Mac.
This program will backup Thunderbird by Year, and it will also make sure that all your emails are safely stored. Even more, now you can now use this program to save the emails from Gmail and Yahoo, so you will never risk losing messages from any program that you're using.
You can keep the data on multiple partitions, storage units and DVDs. Export emails into any format and you can move them very easily from the backup into any program and format you want.More details: Thunderbird Backup By Year With A Unique Program