I really liked the Firefly series when I watched it on Amazon Prime a few years ago, but never did see this follow-up film. I'll have to add remedy that once the price drops.
To me, the film takes all of potential of the show and makes good on it. I don't think Firefly is a good show. I think it's a good idea for a show that never got a fair chance, but that all of the episodes have that "first season" feel that makes them seem a little disjointed and predictable. Much the way people look back at a long running series and conclude that the first episodes weren't great but the show grew into something great, I think that's the case with Firefly. I think it's Whedon's weakest show.
On the other hand, Serenity was outstanding. Everything finally clicked for me there.