The fact that Antivirus Norton is not compatible with Thunderbird,you must configure it manually, perform as explained below:
C:\Documents and Settings\User*\Application Data\Thunderbird\Profiles\xxxxxxxx.default\Mail\Local Folders
- Open Norton Antivirus
- Select Configure > File Systems>Auto-Protect
- Cross check the box [Exclude files/folders selected]
- Click [exclusions] and [Files/Folders]
- Select the folder [Inbox] (without extension) under:
(and / or e-mail account).
- Replace \User*\ with the username of the current session.
- Click on OK and close Norton.