IIRC the main reason Fox canceled firefly was a poor cost to income ratio. Good show, bad marketing (I didn't hear about it until it was canceled!), so-so time slot....and it cost, I think, some 2 million per episode.

Throw in Fox's tendancy to stick with things to long...cou-simpsons-gh...and the fact they like to ditch things that don't hit it big fast...and you've got all the info you need.