Look at Family Guy. Ran for two years then was pulled. Then after becoming a ratings boom on Adult Swim, and having fantastic DVD sales, Fox put it back on the air. They made a joke about getting back on the air if a whole list of other Fox shows were pulled. Firefly was on that rather large list.
The DVD sales for Firefly were astouding. That was one of the reasons Fox ok'ed the movie but as rumor has it, because of George Lucas, the show cannot be revived on TV for seven years. That would six more years from now.
Rumor had it that GL took personal offense to Josh Weaton and his concept for Firefly as that it kinda sorta reminded him of a Han Solo and George wanted, but has yet to ever do anything about it, make a TV series based on Han Solo. These are rumors only though. There are facts that support that George does not like Josh. Comments Lucas made about it him a few years back support that.
Goerge wants to be king of the sci-fi hill. He torpedoed Wing Commander the Movie, (And tv series concept based off of that movie by Glen Larson) - He was semi-instrumental in killing the original BSG - albeit the cost of that show made it to expensive to continue. And rumor has it because he whined to FOX about Josh's show, they dicked him and made it impossible for the show to succeed in hopes of getting George to sign his new Star Wars series and trillagy movies with Fox.
I would bet a pretty nickel that had Star Trek come after Star Wars it too would have become a thing of the past thanks to GL. I am surprised that Stargate and SG Atlantis have survived all these years. They are obviously nothing like SW. Well not until they built the X302's, X303, and X304's.