For many people, it was just the last straw that broke the camels back. You know?
We had suffered oh so many prior cancellations of good shows that only died because they where on FOX, a network notorious for perhaps the most cancellations per-year in the entire industry. Space: Above and Beyond, Dark Angel, so on and so forth.
Firefly was just the one that happened to be, simultaneously, people's breaking point.
Now I myself didn't like the show when it was on TV. Because it was aired out of order, because it was spotty on times, I couldn't get into it. I saw the first few episodes, I watched the premiere, but without the pilot it failed to click with me. I, like many, where thrown in the deep end without a life vest and gave up quickly. It was only later when I, too, got tired of the browncoats lamenting it's loss, that I was challenged -- and provided with the episodes -- to watch it in order, in full.
I did so. I was intrigued by the end of the pilot. I was hooked by the end of the second regular episode, which I'd seen on broadcast but been lost during.
FOX is... all together, bad.
They're a bad news network. They're a bad TV network. They're a bad business network. They're all run badly.
Network FOX seems to be run by impatient 8 year olds. They got spoiled very early on, which is the root of things. Many FOX station carried TNG in it's first run syndication, when they had little of their own programming. It introduced them to sci-fi and very good ratings. Then they did X-Files, which again, sci-fi and very good ratings. Those two shows basically convinced them, sci-fi was super-ratings. So when they put a sci-fi show on that failed to achieve such heights? It got the axe quickly. Because they couldn't give anything a fair shake.
Hell, the only sci-fi show not to be cancelled on FOX in recent years, prior to Fringe, was Dark Angel. And it only got a second season because it was Jim Cameron's, and he had enough pull to keep it around that long. Otherwise it would have died in season 1 too.
The sad fact is, FOX is the only network to regularly give true sci-fi shows a shake; none of the other networks want to touch it. After LOST in 2004, they all tried here and there, but no one made a complete effort.