Steve Brust has written a Firefly novel called My Own Kind of Freedom. It is available for free download here under a Creative Commons license.

http://dreamcafe.com/firefly.html

Thanks, resolute, for the info!

ETA: According to matociquala (Elizabeth Bear) and coffeeem (Emma Bull), Steve Brust has recently had some financial difficulties due to some health problems. Though there is no charge for the novel, it would probably be a nice gesture to donate a buck or two to his fund:
http://dreamcafe.com/donate.html