Not trying to be a stickler (well, maybe I am

), but let me clear up a few things.
First of all... Flock is NOT "new," per se. It's been around for a good two years now... although version 2 was just recently released.
Secondly, Flock is NOT actually affiliated with the Mozilla Corporation (the fine folks who've given us Firefox and Thunderbird). Flock is owned and distributed by Flock, Inc., who have absolutely no ties to the Mozilla Corp.
Flock
is built on the Mozilla Application Framework and it
does borrow most of it's code from Firefox (since Firefox is open-source, this is absolutely legal), but it's definitely not a Mozilla product. It's the same kind of deal as with the fantastic Sonbird music player.
It is great that Mozilla allows this stuff to happen though. They are truly an awesome company.
BTW... Flock is a pretty neat little browser!