Please try the following steps in order (ensure you have a copy of the exported Avast certificate):
1. Close Thunderbird and ensure it is fully shut down (no Thunderbird entries in Task Manager)
2. Temporarily suspend Avast Mail Shield
3. Start Thunderbird, navigate to the Certificate manager and remove the Avast certificates
4. Close Thunderbird and ensure it is fully shut down (no Thunderbird entries in Task Manager)
5. Start Thunderbird, navigate to the Certificate manager and install the Avast certificates
6. Close Thunderbird and ensure it is fully shut down (no Thunderbird entries in Task Manager)
7. Enable Avast Mail Shield
8. Start Thunderbird and retest