There is a free online novel that takes place in the Firefly universe being put out by sci-fi author Stephen Brust. I confess I am not familiar with him, but those who are seem very appreciative of his talents. I can’t tell you how good it is, because after reading the first couple of pages, I had to drop everything to make sure that I got the news to everybody in Oysterland.
If you’re like me, and find the best thing about the Whedonverse is the way he crafts dialogue, I think you will not be disappointed.
The novel is My Own Kind of Freedom, which can be downloaded here.