and IIRC the first movie did not make enough money to garner any sort of sequel, the actors were all on very short retainer for such a thing, and most of them having moved on means it'd take some very concerted Whedon magic to get either the series or a sequel movie.
The DVD Sales on the collectors edition did extraordinarily well (sold out) they released an additional 'Collectors Edition' on HD.
Give it a little more time. Several of the cast have said they are a tight-knit group that would be up for a sequel.
Today:
http://www.fireflyfans.net/thread.asp?b=2&t=33635&newsid=0
Family guy rose from the ashes after high DVD sales, so I wouldn't give up hope on another movie.