Wouldn’t it be cool if the movie were such a success that it spawned a TV series? 
Were not the Buffy fans angry with Whedon, Weldon, whatever, and did not support Firefly?
Beats me, carnivorousplantguy, I never go to fan sites.
In this, we are brothers.
–Cliffy
There was a small but vocal subset of Buffy fans that were pissed at Whedon for the death of the character Tara on Buffy, as well as some that were bummed out at the depressing tone of Buffy Season 6. This subset made it their mission in life to trash Firefly to everyone they came in contact with, and celebrated when Firefly was cancelled. Not that it brought Tara back.
There was also a group of Dark Angel fans who decided to blame Fox’s cancellation of Dark Angel on Whedon and Firefly and made it their lfe’s mission to trash Firefly and celebrate when Firefly was cancelled, Not that it brought Dark Angel back.
And a few anime fans that, based on a single promo for the show, decided that Firefly was a rip-off of Cowboy Bebop. Or Outlaw Star. Or both, or something.
None of these people ever watched Firefly.
Combine this animosity from groups that would normally tend to be fans of the show, with Fox’s abysmal treatment of it, and you get, expectedly, a failure.
Firefly deserved much better.
Well gee, they waited six years and changed the name. How many people really knew it was the TV show they didn’t bother to watch? I’m pretty sure there wasn’t much internet use going on in 1988 (at least compared to these days when everyone and their grandma use it), and they didn’t have DVD sets to remind them what they didn’t like about it, so it was easier then to lure the unsuspecting in. 
Besides, one example is a fluke, not a pattern. Got any more?
I’d be quite dubious about how much these sort of groups actually affect the statistics. As you point out, they’re small. The majority of viewers are casual, mainstream; they don’t make websites about the shows and join mailing lists.