I think I read that there may be three movies. So for now, it looks like the "series" will take place in the movies, not on TV.
What would complicate any return to series television is the fact that the property is now split between Fox (Firefly) and Universal (Serenity). I wouldn't say that it could never happen but it would be more impossible that getting the approval for a movie based on a TV show cancelled after 11 episodes. :-)