This is a tough job because comparing a browser that's at such an early stage of development to the polished product that is now Firefox is not an easy task.
I accept many of Cyril's criticisms about the layout of Flock but let's remember that these are still early days. More to the point however, I don't believe that Flock is trying to compete with Firefox as a whole - I think it's aiming at those that are immersed in social networking and Web 2.0. What I'd like to argue for is the promise behind Flock which I believe lies in its potential for these kind of activities - principally blogging, networking and sharing.
In particular, I can see that for so called 'power bloggers' out there, Flock offers a very interesting alternative. For those using WordPress, Flock makes life much easier by offering unparalleled integration with it. The quill icon in the toolbar takes you directly to the Flock blog editor which seamlessly integrates with WordPress. The blog editor includes quicktags and easily lets you add Technorati tags to your post - all in a WYSIWYG editor. A button to the right of this takes you to the blog 'topbar' where all your latest blog entries are listed and enables you to drag stuff that you are going to publish.
Another factor which impressed me is the integration with Flickr. Just enter a Flickr username and in the all important topbar, you'll see their latest stream of photos. Drag one of the photos into the main browser window, and you'll be taken straight to their main page.
For a blogger or photo fan, Flock promises to be a real treat when it's finished and the various bugs are ironed out. Firefox's secret was in taking antiquated old Netscape, and then simplifying and streamlining browsers. Times change quickly in the internet world however and now browsers are having to evolve to meet the demands of Web 2.0. Let's at least give the Flock team some credit for trying to innovate - things never improve by settling for the status quo.