If you can, see if you can find it at your local video store and rent it out, better to lose a couple of dollar rather than $30.
I introduced 10-12 people to Firefly only around a quarter have really liked the show and of them only one has bought the DVDs.
Sadly, only 1/4 of his friends have any taste.
It's on sale a lot for less than $20, near the holidays it will be about $15-$18.
Plus just ignore the horrible continuation in the movie.
The movie wraps at least one very large ongoing storyline-and you get to see the characters again. Worth it.