Download each e-mail to a separate file before adding to Inbox
From MozillaZine Knowledge Base
Beginning with version 1.5, Thunderbird can be set to download each e-mail message from a POP3 account to a separate, temporary file before appending the message to the Inbox file. There will be one temporary file per message, and the temporary file will later be deleted.
This option was added in response to user complaints about antivirus software deleting or quarantining the entire Thunderbird Inbox when scanning incoming mail instead of taking action on just a single infected message. Especially for antivirus programs that have compatability issues with Thunderbird, temporarily saving each e-mail message as a separate file should make it easier for infected messages to be quarantined, with less risk that the entire Inbox will get locked up.
To have Thunderbird download your POP3 e-mail in this way, go to "Tools -> Options -> Security -> Antivirus" and check the box for "Allow anti-virus clients to quarantine individual incoming messages". It sets mailnews.downloadToTempFile to true. This setting was in the "Privacy" tab prior to Thunderbird 3.0.