I won't post any spoilers, but if you liked Firefly see Serenity this weekend so it can get the best first weekend grossing record it can get. If you'd like to see more of the story, see it a second or third time and bring a friend or two along. Spread the word!
I hope to see it again before the weekend is over--and I'm not a frequent movie-goer. I see maybe 4 or 5 movies a year and rarely see one twice in theater. This movie really is that good.
I recommend seeing the R. Tam Sessions along with the movie. The sessions won't spoil you for the movie at all, it deals with the past of one character that's already been discussed in the series and is made clear early in the movie. I think they add depth to the character and can only enhance your enjoyment of the movie.