I don't find it very productive to ask Firefly fans (or fans of anything) why the show (or whatever) was cancelled. To them, everything was perfect and it was all the network, exterior circumstances, etc. It's fine if people loved the show, I just don't think those are necessarily the people you want to ask about it's faults and the reasons why people didn't watch it.
Believe it or not, I know people who watched the entire series and don't like it, wouldn't watch it again. I know people who watched the first few episodes on TV and then never watched again. It wasn't because of scheduling, they knew nothing of behind the scenes politics, they simply didn't like the show, the characters, the themes, whatever. I'm one of these people, so shoot me.
I simply cited FOX's scheduling decisions as a contributing reason to why it was canceled. It certainly didn't get as fair a shot to succeed as other shows.
That said, I fully accept that there are people out there who didn't like the show. However, I have introduced the show on DVD to at least a dozen people, and not a single one of them hasn't fallen in love with it. Half of these people aren't even scifi fans.