I wouldn't count on there being more shows. At least, not with this crew. As was mentioned, Fox (Fox sucks!) has the show rights locked up. Universal (all praise!) has an option on three movies, the first of which is Serenity. If it succeeds, expect the other two, at least, and I wouldn't be surprised to see more, if the cast consents (and there seems to be a genuine fellowship between them).
Thankfully, unlike Star Trek (the genesis of which is uncannily similar to Firefly, or rather, vice versa), the Firefly actors are much younger, and could make these movies for a good long while.
The movie does lack some of the same character as the show, because the medium doesn't support it as well. The little commercial cliffhangers (which Whedon seems to be a master at manipulating), the long, stretched-out moments between the spaces (Mal putting on the blanket in Out of Gas, for example) - these aren't in the movie the same way. But the humor and the writing is just as sharp, the music is just as beautiful, and of course, the movie medium offers a few things that TV can't.
What an awesome show! Thank you guys for the encouragement to watch the series before the movie comes out. There's so much I would've missed!