Morena Baccarin, the actress playing Inara on Firefly and the upcoming movie Serenity mentioned in an interview that the cast is signed on for doing two more movies.
On the question if there is a chance for Firefly/Serentiy to return after the movie she answered: “You know, that’s something that we don’t really know. We’re right now so happy to just be doing the movie and living in the present with it. We never thought we’d be back together doing this. If anything happens it will happen for a reason and it will be a blessing, regardless of what it is. We’re all still signed on to do three pictures. Hopefully we’ll do more movies and that will be bigger and better.?
You can find the full interview here