I couldn't help but notice tha nobody actually tells you what can improve. I used cross reference photos from http://www.fireflywiki.org and came up with the following: The bridge is a little too high as seen in the front view. The firefly engine goes to a point, where your's does not. A small fixture is missing from the top rear of the engine. Other than those things, great work. Check out the one linked from their website: http://www.reasonablyclever.com/lego/firefly/photos2.htm
Cool. Well I love the show, movie, and comics, so I will always give an 'A' for effort. Not that your Serenity is bad though. It's quite good. 'gotta love the opening flaps, Jayne hatch, and shuttle. Great work.