oh, and check out "leaves in the wind" it was a comic book series which continued on the story. Make sure nothing in your script clashes with this.

I hate to be the @$$ who says it, but, you've got a snowball's chance.
Just being realistic.
Not saying you should give up writing, but writing the sequel to a major movie may not be the best way to start. Get published for something else first, make a name for yourself.

Again, not trying to discourage you, but look at it from Joss's point of view. We was capable to write TV episodes and a movie, which all us fans loved. He could put out a piece of trash sequel and we would all still go see it, even if it got bad reviews. Although I don't know him personally, im guessing he has the integrity not to do so. So think about it, why would he let someone give him a script? (not saying your script is bad, it could very well be better than the original, but why would he use it?) He can write, and he seems to care for his firefly fan base. I am guessing he would do this for himself, and us.

Do yourself a favor. Change a few of the "firefly universe" elements around and try to get it published as an original work of fiction. You might have a better chance of striking it famous. After that, if fortune follows you, maybe you will have a route to get to Joss, and he will take you seriously.

Take my advice for what it is, from a guy who writes as a hobby and is unpublished. I don't have the magic answer, or I would have used it to get published. I spent a few years beating my head against the wall trying to get a novel I wrote in the D&D universe published, but it just can't be done. You have to be one of their writers, and there is a larger story arch, you can't be some guy off the street with his own ideas, even if they don't interfere with their larger story. But who knows, maybe Joss is different?

Hey, good luck, I genuinely hope you can prove me wrong
But if you don't, please don't give up writing, just try something original so you "own" it. Get famous that way, then you don't have to share profits ;)