Comment 57 for bug 115153

This bug very clearly says that the sort order should be by Date.
That's the Date in the Date: header.
That's how this product has worked ever since the beginning of the
Netscape/mozilla/SeaMonkey/Thunderbird product line.
That's how the vast majority of users expect it to behave.
It has recently regressed. This bug addresses the regression.
Using this bug as an opportunity to change the long-time behavior
of the product to suit your personal opinion of a "better" way to
work neither fixes the bug, nor suits the built-up community of
users of Mozilla mail/news clients.
And don't even THINK of saying "but that's how Outlook Express works"!