Probably a few years ago, I suggested about having the option to start Thunderbird with no mail tabs open, much like how Thunderbird used to be years ago before adding tabbed email. Now, it always starts in the same state that it was closed, even if you have several email tabs open. Right now, it is the same as Firefox's option "Open previous windows and tabs"----but CAN'T be changed that I know of in Thunderbird.
From John Nozum