Hi zivilist,
I had come across the same issue with the following setup.
Mountain Lion v10.8.2
Thunderbird v17.0
Avast v7.0 (37781)
I decided to exclude my Thunderbird profile from being scanned by Avast. I do realize this is defeating the purpose of having a virus scanner. If it wasn't for the extremely poor performance I would never opt for this as a solution. Think of it as a temporary fix for the problem you are experiencing.
In Avast Preferences click on File System Shield and click on Advanced and select the following path to exclude.
/Users/Username/Library/Thunderbird - replace username with your short name.
I would also exclude where the EYETV is saving data during live recording.
Hope that helps!