Trouble Deleting Messages

A member in my Thunderbird Help forum has posted this puzzling issue while running a portable U3 version of Thunderbird

When I go to delete an email – maybe I’ve read that mail or maybe it’s spam, should make no difference; however, sometimes the mail won’t delete (but then copies to the trash folder) and sometimes two messages will delete when I only had one selected and intended for deletion. In neither case am I happy with such results. I don’t need multiple copies of mail I want to delete and I certainly don’t want to delete unread, non-spam messages before I have a chance to read them. Incidentally, these problems only occur on the Vista machine. I’m using the portable Firefox and Thunderbird from a U3 flash drive. On rare occasions, when I do email from my laptop (a Toshiba machine running Windows XP home) but still using the portable U3 versions of my Thunderbird and Firefox, I don’t have these problems… but heck, on rare occasions, Windows Vista even does what I want… so maybe it’s a Vista thing or maybe it’s Thunderbird, I just don’t know.