Thunderbird 2: Intelligent reply

Usually (in TB until 1.5+) when you replied to a message that you sent yourself, the TO: field in the reply would contain your account, and the original recipient would be in the CC.
In TB 2 when you reply to a message you sent yourself, the TO: will be filled with the original recipients.

E.g. you find a folder in you SENT folder (most usually) with
TO: foouser@someorg.tld
FROM: myself@someorg.tld

In TB until 1.5+ hitting reply would yield:
TO: myself@someorg.tld
CC: foouser@someorg.tld

In TB2 you get:
TO: foouser@someorg.tld

This is most definitely more likely what you intended.

Great work...