Joss is and has been trying to get it to the theaters. Universal purchased the rights from Fox, so it will most likely be under them. All the cast members have agreed to do it if it gets the final green light. Joss has already wrote a script. Chances are good that we'll see it on the big screen.
While some are skeptical that a cancelled TV show could turn into a great movie, remember he turned the Buffy movie that the suits ruined into a hit TV show that was what he wanted the movie to be. Who says he can't do it the other way around?
BTW, the DVD set is awesome! Firefly fans pick it up ASAP! It was up to number 11 in the Amazon DVD sales rank the other day. Not too shabby for a TV show that Fox thinks no one wanted. Maybe they will back paddle like they are doing with Family Guy.