Unlike Geoff, I have seen Firefly. I actually picked up the DVD of the full series and just finished watching up thru disc 2. And I'm still reading the d20 Future. I'll get back to ya'll on my thoughts on how the S:R and Future integrate. So far my instincts tell me it's more a matter of choosing which elements of Future to include rather than what elements of S:R to exclude.
As to the weapons, the funny thing about the Firefly universe is that the majority of the guns are revolver's and lever-action rifles. Weapons that are definatly *not* covered by d20 Future, but most assuredly *are* covered by S:R.