Firefly was the best sci-fi show on television, but it won't come back. As is the nature of canceled shows, the actors and actresses have moved on and getting them all together again would be very difficult. They might get another feature film though.
I'd be thrilled if they could do something like 2 or 3 straight to DVD films in a row, sort of the way they did Lord of the Rings. But even with all the "Browncoat" support, it's doubtful that the $$$ to justify that sort of outlay would be available. That and the fact that the a$$hats at FOX still own the rights, and I doubt they like the fact that they cancelled one of the best shows ever on TV: The last thing they can allow is a resurection of a series that will make them look like the idiots they are.
Mark