No, Flock is an offshoot of Firefox with social-networking integration (Facebook, Twitter, etc.). Fennec is Firefox Mobile.

Interesting idea, though. If the N900 is supposed to be a mobile computer where people are texting, doing social networking, and making the occasional phone call, then it seems like the more social networking integration the better. But that said, there might be better ways to do it than Flock. Honestly, I didn't even know Flock was that popular.