Having had some time to reflect on F#, I think it's the system I'll use should I ever end up running a Firefly campaign (which I've wanted to do for years). F# is very flexible (good for all that multi-genre-ism), handles tech extremely lightly (just like the show does) and it features the right amount of danger to characters that the series has. Yes, F# doesn't do the "characters die like flies" thing that the Serenity movie did, but that's okay - I wouldn't be likely to use Reavers as Orcs, either.
I'm tempted to run F#iref#ly at the Thursday night group sometime, to try it out. I'm also intimidated/scared to do so, because the coffeehouse we play at on Thursdays is the Seattle Browncoats HQ, and I'd feel like some stranger (or the barista) might wander over and correct me if I break cannon.