How can I backup my Thunderbird email folders and their contents?

Copy the two Thunderbird folders in:

C:\Documents and Settings\UserName\Application Data
C:\Documents and Settings\UserName\Local Settings\Application Data

where UserName will be the username on the computer.

I just search on thunderbird in documents and settings to quickly get to the folders, then drag and drop them to my backup drive.

Create seperate folders on the backup drive for each so you don't get them mixed up, for example, one named "application data" and the other named "local settings application data"

To duplicate to another computer, just install Thunderbird, then copy the folders to the correct location, either username\application data or username\local settings\application data