I've started having problems with Thunderbird demanding certificates before and after following steps here http://www.avast.com/faq.php?article=AVKB91#idt_02
for exporting and importing security certificates. Has similar problem in April but the export/import routine fixed it. Now in order to send or receive emails I have to approve security exceptions repeatedly. Something in avast must have changed to trigger this...some tweak to the email scanner.
I recently upgraded thunderbird to V24 after the problem started and was asked for certificate when upgrade completed indicating upgrade didn't improve situation.
Avast current version is 8.0.1497
Suggestions and insights appreciated