Based on the review at
http://www.mactips.org/archives/2008/02/02/flock-awesome-web-browser/

I downloaded and installed Flock 1.0.8 on a PowerBook G4 running Leopard. On first launch, I opted to import my settings from Firefox. The result was a Flock menu bar, but no windows. Flock would not display a new window, nor would it quit. I forced it to quit, dumped that instance, and re-installed. I checked the Library/Application Support folder, but nothing for Flock. I launched again. This time I got a "script taking a long time to execute" dialog, which I dismissed. Then I got "You're closing 5 tabs" dialog (the Home bookmark has 4 tabs), and when I canceled that dialog, Flock crashed (reported to Apple). Now there's an App Support folder. I trashed that and relaunched, this time opting to import nothing. The browser opened and appears to be functioning OK.