The basic problem here is that not enough people knew about Firefly. Fox screwed up the original airing schedule so badly that only a handful of people ever saw it. Those that did manage to keep up with the horrible scheduling generally LOVED the show, but the average person simply didn't know about it.
Then, the series was made into a movie, but due to the fact that Fox owned the name "Firefly", that word could not be used in any of the marketing for the movie Serenity, which meant that many people weren't able to connect the two. And, let's be honest: the Serenity marketing was clearly aimed at existing Firefly fans more than at the moviegoing public in general, and the existing fan base was just too small numerically to help this movie out.
Had the series been able to go a couple more seasons, it would have been able to build a big enough following to support a movie, but sadly, that didn't happen. Really, Firefly belongs on TV anyway; you need all that screen time to tell the story the right way, and if you try to cram it all into a movie, it seems rushed and you don't get the needed character development.
Unfortunately, FOX isn't about to sell the rights to Firefly, and that effectively kills the whole Firefly concept.
At least I was able to pick up Serenity tonight, with $10 off for also buying Battlestar Galactica Season 2...
-Troy