Don't know of any official stuff, but I offer this advice:

-Change the rules for subdual damage. In D20 Modern, unlike D&D, for some reason it's mathametically impossible for one unarmed human to knock out another unless his strength is literally epic. Doesn't much matter how you change it, but Firefly relies way too much on fistfights not to.
-When someone rolls a 20 with a firearm, make it automatically a critical (instead of a 'threat'). This makes combat just slightly more deadly and occasionally hilarious.
-Allow PC's to use the Treat Injury skill (or whatever it's called) without a medkit to stabilize a dying person (usually a medkit is required). Medicine in D20 is by and large good enough to simulate the fact that on Firefly you can survive almost any injury if medical care is swift and skilled, but it's WAY too easy for folks to die before the medic gets there.
-You will have to massage the wealth bonus system some, as it's designed to handle finances much more complex than those in Firefly-- and doesn't deal with splitting a take very well. My group did find a solution, which I can post if this is of concern to you.
-Encourage your crew to incapacitate people without killing them, and make this more viable by having non-mortally-wounded NPC's give up or collapse from the pain of being shot. My crew just left this godawful trail of bodies...
-Make sure that the captain is a PC (sooo boring otherwise), but make sure it's somebody you can trust to be gutsy but not insane. Cowardice makes for a very boring game, but recklessness should make for a very short one. Pilot and mechanic should also be PC's; otherwise it's mostly you rolling dice. Medic can safely be an NPC, but let them do what they want. My crew had a cook.
-I'd discourage using the crew from the series. You'd be surprised at the awesome characters and backstories folks come up with, and the 'Verse is more than rich enough to support it.