Nice enhancement request but that would mean (for it to really work) that your whole TB DB is completely encrypted. It will make TB slow. It will be better if you secure your computer instead. Make it lock frequently (if you haven't used it for 5 minutes) and/or encrypt your TB folder usually at C:\Users\YourUser\AppData\Roaming\Thunderbird. Don't forget to backup your encryption keys and that folder in case you ran into trouble.