I gave up on that and switched to Avira, another freebie.
I have used that for 3 years now with no problems, if you can't sort out AVG may be worth trying something different.
Have you tried this
<<In Thunderbird's main menu, go to Edit, then Account Settings, select OpenPGP Security, there you will find the message composition default options.
I think you have to un-check the option: Encrypt messages by default >>
HTH