Firefly fit the constraints of TV so well, I worry that it won't translate well. It is also very detailed and ongoing...not generally the flash that can be folded into two-hour movie and then left with nothing for a few years.
I know what you mean. On the other hand, Whedon has been very successful at writing for movies as well as for TV. (He was involved with writing Toy Story, for example -- one of the better movies ever done IMHO.)
So I think he knows the differences, and will probably do very well.