I’m using Thunderbird as my email client, and lately I keep getting security alerts asking me to confirm certificates. I have exported the certificate from Avast to Thunderbird as requested, yet still keep receiving these alerts. What’s going on with these?
no problems like this when using webmail 
have you done it like it say here http://www.avast.com/en-eu/faq.php?article=AVKB91#artTitle
Yes, I have done all that, and it still pops up. When I click confirm exception, it places that certificate back under the Server tab, where the instructions you referenced said to delete it. I did so, but it comes back.