If your someone who likes Firefox features but finds Firefox too unstable or even possibly too insecure then Flock is for you. Flock is pretty much Firefox without the stability issues and without the security issues. However there are as many downsides to choosing Flock over Firefox as well.

First Flock will always get updates after Firefox. So all those security patches won't come until Firefox users already have them. Flock also is secondary when it comes to the latest Firefox features. Flock users won't be able to experience the latest Firefox features until months after the official Firefox release. While Flock is compatible with Firefox add-ons, it's not compatible with their themes. Also don't expect every Firefox add-on compatible with the latest version of Firefox to be compatible with Flock. Flock is based off Firefox 3.0 engine, not 3.5 so there is a good chance 3.5 add-ons will not work with Flock.