I have discovered that, while downloading new messages, there is a file "newmsg" that is saved on the hard drive in the current user's temp directory (\Documents and Settings\[user]\Local Settings\Temp\). It seems to be a placeholder for the current message being downloaded. When the message is downloaded, "newmsg" is removed and apparently imported into the Thunderbird mailbox. then, "newmsg" is recreated for the next incoming message. I have opened up the "newmsg" file in Notepad, and it is the full e-mail, headers and all!
This side-effect is directly related to the setting in Tools -> Options -> Privacy -> Anti-virus -> Allow anti-virus clients to quarantine individual messages. If the setting is disabled, newmsg is not created. Also, individual messages cannot be scanned by the anti-virus, which means entire mailboxes could potentially be deleted by the anti-virus client.
If an adversary gets access to the hard drive from the computer that Portable Thunderbird was run from, it might be easy to recover the contents of all of the "newmsg" files. Seems like a huge security risk.
The solution to this is to store "newmsg" locally on the USB flash drive Portable Thunderbird is running from. So far, I have not found a way to change where "newmsg" is put. I propose that the location of "newmsg" be configurable with a setting in the config editor (about:config) or elsewhere so we may put it locally on the USB flash drive Thunderbird Portable is running from. Or, as a feature of Portable Thunderbird, force "newmsg" to be created locally on the USB flash drive. Etc...
Also, what other personally identifiable files are put on the hard drive? If there are more, these also need to be addressed so they are put on the USB flash drive.