Backing up Thunderbird is very easy.
*Tip – You should move your Thunderbird profile folder to a different partition than your OS partition (preferably another drive). This minimizes the risk of anything happening to those files in the first place. See: mozillaZine – Moving your profile folder I generally keep mine in the same place I moved Libraries to (My Documents for XP & Vista Users).
All you need is Mozbackup. (download the zip file, not the installer)
To manually backup Thunderbird see: mozillaZine Profile backup