A lot of people really like Flock’s theme. I’m not much of a fan personally preferring the more simple Firefox default theme, but I can see why people love Flock’s theme.

FloFox is a Firefox theme which looks like Flock (and does a really good job of it). You can see screenshots at the MozillaZine thread.

Note: You can get an updated version of the theme here. This updated version is based on the UI in the Flock public beta.