Mozilla Seamonkey is the revival / continuation of the original Mozillasuite, which was left stalled at 1.7 / 1.8a3, having been superceded by what were originally the sideline component versions, Firebird and Thunderbird (and I use the Firebird name, as that is what it was called at the time).
The other functions of the suite were less well handled, with the calendar finally getting some recognition after much neglect -
http://www.mozilla.org/projects/calendar/ - The "bird" name is Sunbird, but there is another split as well.
As for the "Composer" part, that is really gone, the baton for that carried forward by NVU -
http://www.nvu.com/index.php - There is no known "bird" name for the editor component.
Seamonkey is for those who were dismayed at Mozilla plumping for the "birds" and neglecting the suite, and kind of reverses the situation where the "birds" were passengers of the suite development.