Receive message, press reply, write something save and close draft. Re-open draft send it. No \Answered flag is set on message which I'm originally replied. Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b10pre) Gecko/20110117 Thunderbird/3.3a3pre

It does set flag correctly if I reply on message and then send w/o saving to draft and closing composing window.

Replied mail is held in IMAP mail folder, and draft folder is IMAP folder? If so, it's probably regresson seen in bug 522336 comment #29, because your STR is "Reply, Save as draft and exit composition, and Edit draft then Send. Because "IMAP draft folder case or local drfat case" is still unclear in bug 522336 for both original bug report and test case which confirmed that bug, I believe duping of this bug to that unclear bug will merely produce confusions. Duping to bug 530528(for apparant IMAP case, for Tb 3.0.1pre).