Firefly isn't a great perk for PvP. Unless you get some kick out of your opponent disintegrating into a fiery blaze upon a precision kill then I do not see any use of Firefly in the Crucible. Hidden Hand and Unflinching are the way to go. The real reason why 1K Yard Stare is so prominent is because other High Impact/Res Sniping Snipers have limitations. One is Exotic (Black Spindle) meaning no Thorn, MIDA, Red Death, Bad Juju, TLW, TFC for you. One is Hunter Exclusive (Stillpiercer) which cannot be equipped on Warlocks or Titans The other (Eirene) has horrible scopes and can only be obtained through the Gunsmith rather than the RNG of engrams, Vanguard packages and Nightfalls.