If you would have listed Star Wars(the whole force aspect) I'd agree. But why is Firefly blurring the line?(Don't know what S:AoB is however.)
Firefly's society seems improbable. Granted. Especially the mix of old firearms and space travel is rather wierd. But while improbable it's not impossible. The interstellar travel is somewhat lacking as it completely ignores the distances and the need to go FTL to reach their destinations.

But the only thing that could make it into fantasy is the Psi Corps, erm I mean River's abilities. And Firefly is at least trying to give a plausible explanation for it.