Many reasons why Fox gets the blame..
a) Firefly is a chronological show.. while each episode is mostly self contained they do build on each other and because Fox run them in a disorderly fashion people were scratching their heads when some events were referenced which they didn't see
b) Serenity was the pilot episode and got shown last
c) The Friday slot of death.. no show save X-Files made it if it was in that slot
d) Not as much advertising as should have been.. if you are producing or paying for an expensive SF show make sure everybody knows about it
e) When all these factors led to unsatisfactory viewership numbers Fox just gave it the ax without trying other slots or something else..
f) Fox repeatedly hampered attempts to release this show to other stations.. it was their property and they clung to it as if it was worth something to them
g) DVD sales were gigantic and so were reviews of the show but Fox never considered a re-launch.. there obviously was interest but Fox ignored it
h) In an unparalleled move Firefly even got its own movie (and not a direct to DVD kind of thing).. while not that successful to warrant sequels it still was a success
All considered Firefly fans were pissed that the show was never properly treated by Fox so it couldn't have a chance to build an audience besides hardcore SF fans. So naturally it slumped and immediately got the axe.. as if Fox execs just waited for the opportunity to burn it down.
This is what pisses of Firefly fans.. if the show had proper support like other shows and failed they would still bitch about it but it would have been less.. Firefly had its chance and failed. But as it stands it was sabotaged by Fox from the beginning.