It would take lots of money, and a huge audience. Not to mention the participation of a lot of creative people that have moved on to other projects.
Considering Firefly was already canceled and Serenity wasn’t even a big enough hit to warrant a sequel, you have a hard road ahead of you if you’re trying to bring Firefly back. It’s a shame it never caught on, it was one of my favorite shows ever.
But if you’re a Whedon fan, there’s a new show coming out that he created called DollHouse. I’m definitely going to be checking that out.