Basically, it comes down to this: I hate scifi.
Not that I'm particularly fond of any genre; I can't stand the idea of liking or disliking something simply because certain elements in it line up with other films, books, whatever. Suffice it to say that I can't stand scifi conventions (as in genre, but I guess that sort of applies to expensive nerdy get togethers). I don't like bumpy foreheads of the week and I don't like the Weird People With Their Weird Custom of the week. It's mindless drivel, and there's little difference between it and CSI melodramatic Crime of the Week.
Firefly's different. It's well written, well acted, and it's a show about people and what the future does to them. It isn't about diplomats or military leaders or starship captains, and it really stays the hell away from all the other dull and witless fanservice that has defined the genre since Star Trek. In a lot of ways, Firefly and Serenity make up some of the first truly good scifi to come out of America and reach beyond being simple genre pieces.
I do agree with crac, though; while the movie does do a good job of setting up the characters and the world (and a friend of mine who saw it with no knowledge of the series came out pretty much in love), there are jokes you won't get and things you won't quite feel without the context of the series, which is eminantly worthwhile in its own right.