They are in a distant future where the captain of the Space Ship Serenity is a wandering veteran of a failed revolution of independent free planets against the evil "Alliance" of dominant worlds. One of the hooks to make it different was that the characters peppered their speech with Chinese phrases and oaths as if at least sometime in the past (our future), it was the dominant culture. I felt when I first saw it that it was an attempt to groom us for our eventual downfall.
Josh Whedon also created Buffy the Vampire Slayer among many other projects.
Ok, I tried to watch Firefly because it has a very loyal following, but I just didn’t get into it. Hyperdrive and Stan Against Evil are more my speed.