The series never did well, because the network pooched it. As Joss Whedon put it, "[Firefily] was instantly hailed by critics as one of the most cancelled shows of the year."
I'm not a huge fan of the TV series, but to watch the story in the order it was supposed to be aired makes a huge difference. Mr. Whedon understands that viewers like long, complex story archs, and built the show around that. The network decided it knew better...
The movie, was, (to coin a nerd-pun) light years better than the TV show. Joss really out did himself, and even if you haven't seen Firefly, the movie works completely on it's own, and is still a frickin' great ride.
If you're familiar with the TV show, it makes the movie just that much better.