You should routinely backup your Thunderbird mail folder if you store your email messages on your computer.
Messages are stored on your computer under the following circumstances:
If your computer's hard disk malfunctions, you will lose all data stored on it, including your Thunderbird mail folder. To prepare for such catastrophes, you should routinely copy your personal data, including your Thunderbird email folder, to an external storage device, such as a USB flash drive or external hard disk.
The following instructions will assist you in backing up your Thunderbird email folder.
First, locate your mail folder by navigating to your Thunderbird profile using Windows Explorer. The default location is:
C:\Documents and Settings\{your-Windows-login-name}\Application Data\Thunderbird\Profiles\{your-Profile-name}\
The default profile name is a string of 8 random characters followed by ".default".
For example, in the screenshot below, Windows Explorer displays the contents of the Thunderbird profile for a person with a Windows login name of "login" and a default profile name of "1gb5e5tz.default".
Within your Thunderbird profile folder, locally stored messages are in the folder named "Mail".