Mozilla’s Thunderbird is a great email program (we use it!) but it doesn’t have any built in backup tools. If something goes wrong, or you want to move your profile to another computer, you’ll need a backup.
First of all, make sure Thunderbird is not running.
Open Windows Explorer. In Folder Options, enable “Show Hidden Files, Folder and Drives” .
Then browse to C:\Users\yourloginname\AppData\Roaming, where you will see a folder named Thunderbird.
The Thunderbird folder contains all your Thunderbird settings and address book, as well as all your emails, so it is this folder that you need to copy to your external drive, desktop, or wherever, as your backup. It’s likely to be a very large amount of data so expect it to take a long time!
When you’re done, don’t forget to disable the hidden files view in Windows Explorer by selecting “Don’t Show Hidden Files, Folder and Drives”.