Just because some cancellations were due to Fox's incompetence doesn't mean all of them are.
Fox did enough wrong with Firefly to obscure the real problems with the show, which was that it was a space-opera-cum-western with pro-Confederacy overtones with little appeal outside of a small cadre of "science fiction nerds who like that kind of thing."
Fox didn't commission Firefly because they were enthusiastic about the concept, but because they needed to kill the absurdly expensive and rapidly tanking Dark Angel, but do so without angering James Cameron. Commissioning a replacement science fiction show from a rising star (as Whedon was at the time) was a move that would look good enough on paper that it wouldn't appear to be a slap in the face to Cameron.
Firefly was commissioned for all the wrong reasons, it was a flop, subsequent events, such as the disastrous box office figures for Serenity (a low to medium budget film whose box office receipts - half of which are kept by the cinemas - was lower than its budget), show that the concept had virtually no appeal, and it's really only a combination of Fox's screwing with the schedule - understandable given they realized early on they had a lemon on their hands - and decent DVD sales - that's created the myth it was somehow a superb, popular, series that would have been as popular as Buffy had it just had the same level of support.
Fox also put pressure on Whedon to do things to early episodes of Dollhouse that also caused a run of barely watchable episodes until Episode 6. However, the show getting back on track, losing the abysmal Terminator ball and chain, and being nurtured by Fox did nothing to improve ratings, despite switching from barely watchable to arguably the smartest show on television. Sometimes concepts just don't work. Had Dollhouse been canceled by, say, episode 11, there'd have been enough people who had become fans of the concept for it to develop a similar cult following, who'd have likewise assumed that the problem was with Fox rather than the show itself.
But we know that isn't the case. Despite becoming good, the ratings continued to drop. Sometimes shows just don't have appeal. Sometimes the concept just isn't strong enough to attract viewers. Dollhouse was a great show that fits that description. And, alas, Firefly was a somewhat poorer show (not a bad show, don't get me wrong, but I've never understood the obsession with it as "the greatest science fiction show ever!" It's not even science fiction, it's a space opera damn it!) that also fits the description.
People have the cause and effect backward. Firefly didn't fail because Fox messed around with it. Fox messed around with it, and in the end stopped caring about it, because it was a failure.