"Love it, or hate, Gmail introduced millions of people to the
concept of fully threaded email conversations.
It's a concept that Mozilla is now delivering to its Thunderbird
email users by way of a the Conversations add-on.
Conversations this week got it's first stable release, tagged
officially as version 2.0 (now to version 2.0.3 for some
incremental bug fixes). The real push with this add-on is that it
is fully integrated with the new Thunderbird 5.0 release which came
out last week.