:)Maybe good news for some of you....I had this problem with my new computer. It has W7 pro on it, and I first installed Thunderbird, and then installed Avast several days later. Thunderbird worked perfectly until I installed Avast, and the symptoms were that Thunderbird was not able to connect to my mail server, either to receive or send messages. It took me a while to even realize that Avast might be the problem. Then I found this thread, and started adjusting things, as suggested. I was able to export the Avast certificate and import it into Thunderbird, but this did not solve the problem. Then I noticed that in reply #38, by stduc, he/she mentioned that the Thunderbird certificate needed to be deleted and re-imported from Avast. Well, I deleted the Avast certificate from Thunderbird, and then tested Thunderbird, and it worked! Well, sort of. Thunderbird asked me to confirm certificate exceptions, which I did, and THEN it worked just fine, with Avast doing its scanning like it should.
So I have this diagnosis: Avast found the Thunderbird folders when it was being installed, and inserted a certificate, which didn't work. I have other computers where Avast was installed before Thunderbird, and I did not have this problem. So now I suppose Thunderbird has created whatever certificate information it needs, and it has done that right. As far as I know, everything is working just fine, and as designed to work. I hope this helps someone!