Hi Daniel, welcome to the forums.
visit the
User's FAQ thread, it will give you a lot of useful advice.
Run the Mail Protection Wizard, make sure that Thunderbird is closed before running the Mail Protection Wizard. Also re-read the avast help file - Setting up the Mail Protection.
A search of these forums for Thunderbird will no doubt return many hit as this has been a previously discussed topic.
HTH David
Edit: Oops - You will have to set up Thunderbird manually, I was thinking Mozilla Mail - Manual setup is in the user faq and avast help file though.