"Earth-that-was could no longer sustain our numbers, we were so many. We found a new Solar system — dozens of planets and hundreds of moons. Each one terraformed — a process taking decades, to support human life, to be new 'Earths'. The Central Planets formed the Alliance. Ruled by an interplanetary parliament, the Alliance was a beacon of civilization. The savage outer planets were not so enlightened and refused Alliance control. The war was devastating, but the Alliance's victory over the Independents ensured a safer universe. And now everyone can enjoy the comfort, and enlightenment of true civilization." – Serenity
Firefly has been described as a "space western". It takes place in the 26th century, after a long war between the Alliance and the Independents, which somewhat mirrors thematically the American civil war. In this new solar system, the Central Planets are high-tech and highly civilized and ruled by a single entity called The Alliance. Settlers spread to outer planets, often with just the bare minimum to possibly scratch out a living from the new soil, again, not unlike many parts of the New World had been settled so long ago.
Then there was the war. The Alliance wanting absolute dominion over every planet in the 'verse, while the Independents fought to remain, well, independent. It was a brutal victory for the Alliance, leaving the outer planets now under Alliance control, but not much better off for it.
In these planets, along the outer rim, the frontier way of life is much harsher, with survival being priority one. The general feeling out here is very similar to the American Wild West.
Except, of course, with spaceships.
