I've only read two fics for Firefly. They were both pretty good, but I'd recommend you read a bit first to see if they are what you have in mind.
A Life Less Extraordinary A good Doctor Who/Firefly crossover, that somehow works. Set after the movie I think. It's been a while since I read it. It is just after the Doctor was separated from Rose and after runaway bride episode I never saw and hasn't gotten another companion yet.
The other was:
Browncoat, Green Eyes A Firefly/Harry Potter crossover that is very good. Features a Harry Potter who lived his life on Earth, but kind of got really really bored. So sealed himself up up in a ring he gave to the Weasley family so that if things ever got bad they could call him out to show up the next Dark Lord. Too bad he wakes up centuries later to find humans have fled Earth That Was, and magic is... gone?
These are both nice long fics, 100k and 200k words respectively.