Not a bad start and I do hope to see more, as a starting chapter it's not overly amazing since it copied canon so much but at least the set up isn't to bad. The few firefly cross overs out there generaly have real trouble finding valid reasons for why any one else would join up with the firefly crew all the sudden and generaly being a fellow experiment with River is one the better places to start.
That being said the key point I would have to say to make this story truely stand out is complete it, far as I know there are only maybe one or two HP/Firefly fics out there worth anything that are completed. Almost all them go dead within a few chapters.