I've been thinking the same thing (about Firefly -- I've been in some good Sci-Fi campaigns). 'Firefly' is well-suited to RPGs, I think. The major problems I've encountered in running or playing in Sci-Fi campaigns have been largely due to money and travel.
1. In modern and post modern settings, scarcity of an item typically should not be an issue, while in fantasy settings, swords, etc., have to be hand-crafted at a significant effort and expense. From a player's perspective, not being able to find exactly what they want makes sense in a low-tech campaign, but they (rightfully) get a little skeptical when you tell them that there isn't a single heavy blaster rifle on the planet.
This quickly leads to a situation where either everyone has every piece of equipment their hearts' desire, or the DM is forced to constantly work to keep the party poor and items hard to find. The tech-level system in Traveller makes this a little easier to deal with, since they will know ahead of time that if they are going to a TL5 world, they won't be able to find certain things. This dove-tails nicely with the Firefly setting, which has the relative paucity of technology on the fringe as a central theme.
2. A large part of low-tech campaigns will be focused around travel -- random encounters, tracking enemies, going from town to dungeon and back, etc. This has a couple of side-benefits:
-- it keeps the flow of time plausible (ie No, you didn't go from 1st to 2nd level in 2 days -- there was weeks of travel involved)
-- it makes keeping the party supplied a challenge (sure, you can run back to town for more healing potions, but it will take you a week, and the goblins you killed will have been replaced)
-- it let's the DM keep the world at a size he can manage. You don't have to worry as much about players heading for parts unknown, and you can delay them enough to give you time to prepare for it.
In Sci-Fi settings, where players feel free to hyperspace about on a moments notice, this can be a problem. Need more med-kits? Hop on your landspeeder and be back from town in 20 minutes. Have a yen to visit Tattooine, but you're halfway across the galaxy? No sweat -- a ticket on a space transport is cheap, and it only takes a couple days to get there.
In a Firefly setting, ships are scarce, and it seems like some planets go weeks or months without an off-planet visitor. This would let the DM keep a little tighter rein on player movement, and help keep the campaign focused.
Maybe that's enough rambling for now....I'm looking forward to checking out the Traveler d20. I've been disappointed with SW d20, and I've not read a single thing about d20 Modern that makes me at all interested.
