So, I've been trying to get Norton to deal with Thunderbird for a while now. Every week it would scan my Thunderbird email and quarantine the entire inbox. This meant I needed to restore the inbox.
The instructions at Symantec said to simply excluse the mail folders - which I did - to no avail. Today I finally found the answer. You see, even though you may tell the system to exclude X, you have to configure the exclusion in your scheduled scan as well.
Probably obvious to most folks, but it wasn't for me.
Switch to AVG Antivirus. Use the personal email scanner. It will scan incoming mail BEFORE it gets thrown into your inbox.
I've been very happy with Avast. Free, effective (as far as I know :-), and plays nice with Thunderbird.