Firefly was a TV series that got cancelled after one season on Fox and Serenity is a movie that takes off from where the series stopped.
Firefly follows the adventures and misadventures of the space freighter Serenity and its crew led by Captain Malcolm Reynolds. It takes place in a future universe where humans have colonised a remote galaxy with many habitable moons and planets. The writ of the ruling Alliance doesn't run very well in the remote outer planets where Serenity's crew ply most of their illegal trade. The remote planets are portrayed with a wild-west feel, with run-down bars, brothels, local tough-guys and whatnot. The series has a mixture of western and space-faring adventure feel to it that is very appealing.
Captain Reynolds is very much an updated Hans Solo, pirate-with-a-heart-of-gold kind of guy. The rest of the crewmembers are also portrayed very well. Some of the characters are almost cliched but they are well written and well acted so it actually works out very well.
The series has a general upbeat feel to it, in a old-fashioned swashbuckling action-adventure kind of way. I very much enjoyed it.
The movie takes off six months from the last episode and is as good as the series episodes.
The series (and the movie) did not do as well, which is a good reason why one must not make popular reception the sole metric for choosing a movie. They have however, managed to pick up a cult following and I hope they do well in the DVD market.