Had the same problem as Wagner above. Fixed by doing the process in the msg you linked to.
I would like to suggest to Avast:
1. Adding an FAQ or relevant help section as soon as possible (especially updating the help on the SSL tab to indicate the manual cert import procedure for TB).
2. Having avast mail scanner check to see if Thunderbird is installed and has the certificate imported. The best place to do this is on the Mail Scanner SSL tab itself.
3. Investigate as to why the certificate was not imported into TB (as what happened in my case ).