Blame it on the theme song. See, audiences don't mind sci-fi Westerns as long as they don't really push the Western part (you rarely saw any horses on ''Star Trek''). But ''Buffy'' mastermind Joss Whedon had to go and open his show with an ambling country ditty...and it was canceled 13 episodes in. Too bad, because Firefly: The Complete Series was a smart, satisfying genre reinvention, about a mismatched band of hired guns flitting from outpost to outpost looking for honest (or dishonest) work. The cast was strong, led by Nathan Fillion's roguish captain, the writing renegade-tight; Whedon was trying something new -- it's a shame he didn't get to finish. EXTRAS A fan's treasure trove: Three never-aired episodes will go over like crack with the ''Firefly'' faithful, as will the many commentary tracks (advice: Focus on the ones that feature Whedon -- they're funny and pervasively knowing). The docs and deleted scenes are worthwhile too, but the set's bittersweet vibe is at its most affecting on the gag reel. It was made for the cast and crew's Christmas shindig, which ultimately ended up being the series' wrap party instead.