SeaMonkey has flourished since Moz declared it a "project" and not a "product"
I guess the wheels started coming off the rails at Mozilla even before the rapid release cycle madness kicked into gear, where ratcheting up version numbers took precedence over actually fixing broken things and refining the software (product or project) to make it better.
FF is becoming (has become?) Chrome (Chromium) from another company, so it's no wonder, considering that that other company doesn't have its own email client (let alone suite), that Mozilla is eager to shed yet another differentiating factor...
Personally, I've been a SeaMonkey user since Netscape Communicator 4, and have been generally quite pleased with the focus of the Team (except when there's been too much of a rush to "keep up" with/"catch up" to the insane FF release cycle). Hopefully, TB can do as well (for my money, though, scrap the current TB code and fork the mail client off of SM...again).