"Never heard of Firefly."
I weep for you. I don't even know how to explain how awesome Firefly is. I've seen most "really good" TV shows. I watched the Star Trek franchise start to finish. I was a huge fan of Arrested Development. I've seen every episode of Battlestar Galactica multiple times.
If you added up those three shows and combined it into one, epic show, it would not come close to achieving the brilliance of the short lived 14 episode run that Firefly had. Firefly tops almost every single "Canceled before its time" list there is, and fan reaction was so intense when it was cancelled, they did a follow up movie, Serenity.
The long story short is this: Firefly failed because of one reason, and one reason alone: FOX. Just like they did with every show that required a modicum of intelligence to digest, (Arrested Development, Terminator: TSCC, or just see http://www.toplessrobot.com/2009/08/the_20_greatest_shows_canceled_by_fox_before_their.php) they stuck it on Friday nights, and watched it dither into the night. What's more, they only aired 11 out of the 14 episodes, and aired them out of order, which for Firefly, was a huge problem.