Flock was a Gecko-based (Firefox/18.104.22.168) web browser designed to interact with web services such as photo sharing services, blogging services and so on better than normal web browsers by providing extra features just for specific services. It was basically Mozilla Firefox with some extra features. The default bookmarks included were: MySpace, MTV and YouTube.
It's over, Flock is dead
Flock was announced dead in April, 2011.