When you click on the 'New Message' button, Thunderbird then gives you a new window with the Compose New Message screen.
But say you clicked that accidentally, or you decide you don't want to send a message ...
Then you click on the X in the upper right to exit. Thunderbird pops up the Save Message reminder, saying "Save this message to your drafts folder (Drafts) and close the Write window?" with the options Save, Discard Changes, or Cancel. Then you have to click one of those options before this window goes away.
So I would suggest a possible enhancement to Thunderbird:
If nothing has been entered in the Compose New Message screen when the user clicks the X exit, then exit without requiring confirmation via the Save Message popup. This would be faster for the user, and reduce the small annoyance of asking them to 'Discard Changes' when in fact they have not made any changes.
It would be best if this happened when the user had made no changes to any of the fields (To, CC, BCC, Subject, or Body Text). But acceptable if it happened only whenever the Body Text had been left blank.