I really liked this movie when I saw it in the theater. At that time, I’d barely heard of Firefly and I was just interested in the movie because it sounded like sometime I would like. Therefore, I think you can understand and appreciate the movie without the series.
Last fall, a long time after having seen Serenity, I started watching Firefly on a whim. I fell in love with Firefly, the western music, the characters, Serenity, and everything else that is wonderful about the series. When we reached the end, the next step was to watch the movie again. I really liked the movie again, although I think I like the series more. Nevertheless, the movie maintains most of the elements I loved about the series, minus the western music. If you are interested at all in eventually watching the series, start with the series and watch the movie last. If you are worried you won’t have time or you won’t finish, just start with the movie. You’ll want to watch the series when you’re finished. :)