Flock is built on the firefox backend with a few nice additions.
1. WordPress is integrated into it (im using it to post this blog)
2. Social networks like twitter, facebook and myspace are also integrated in a sidebar for easy access.
3. Gmail and yahoo are in there too
It takes firefox and makes it more integrated way with socal networks and email. A few drawbacks are that everything that I mentioned above can be implemented better but this my come with time. For instance it has support for video feeds but doesnt sort it by when it was uploaded (I have feeds from Youtube and it shows me videos from oldest to newest). It may be built on the firefox backend but it still hasnt support for windows media files. All that being said it still adds something new to the browser wars but maybe should just be an addon to firefox and not a separate browser