Are you using a portable version of Thunderbird perhaps? If you are, then you must do some manual configuration because the Avast installer does not propogate any installation parameters to the portable Thunderbird email program. Follow the instructions below:
Open an elevated command prompt (eg run as admin) and execute "certmgr.msc" (without quotes). Now find the Avast Root Certificate and export it to a common directory (ie, downloads, documents, etc):
Once done, open Thunderbird and navigate to the Certificates. Delete the current Avast Ceritifcate (if shown) and continue to the next step:
Now, import the new certificate that you extracted via running certmgr.msc into Thunderbird as shown below;
And finally, edit the newly imported certificate as shown below:
Also, I'm guessing this might fix your problem even if you are running the installed version of Thunderbird. Good luck!