First, you have to make sure that the soundtrack you're buying is not so recognizable to your players that they are taken out of your fictional world and into the fictional world the movie or video game created. In this respect, for your Firefly game, don't use the Firefly or Serenity soundtracks - everyone who cares about Firefly has already seen every episode a thousand times and is certain to recognize and respond to music cues from it. Instead, look for Western (or Eastern) themed soundtracks to get the feel of the setting/action across (you are running your Firefly game like a western and not like a science fiction game, right? right) without removing them from the world of your game.
This leads to my first recommendation from the sale, Thomas Newman's "Skyfall" and why you should be careful with it.
|"Skyfall", Thomas Newman, $5 in Februrary|
|"Moneyball", Mychael Danna, $5 in February|
|"Hook", John Williams, $5 in February|
I really do recommend the Amazon mp3 store - you get free downloads of the unlocked mp3s from the cloud to back up or install on any device you like. I'll give you some more soundtrack analyses with some specific campaign ideas another time!