Well, somewhat concrete. I was doing the first part of crota on normal with a buddy when another friend asked if I could wait at the end. He needed the drop. Rather than sitting a top a rock I decided to hold the bridge. Armed with my saterienne rapier, icebreaker, and some sort of heavy machine gun I held that bridge like a champ. Level 30 thralls were one shot by my scout rifle. Ice breaker was used on knights and the machine gun came out for an occasional ogre. How many kills did I get? 993. How many were precision? 502! Firefly nearly [b] doubled[/b] my kills. Now I know that the enemies were bunched up on the bridge but still. Show me one other perk that at maximum effectiveness will double your kills. Tldr firefly is love firefly is life. It's the most important perk in the game. Hands down. On a somewhat related note, this is the survival mode we've always wanted. On normal you can hold the bridge alone or with 6. Adjust the difficulty with your gear. See how long you can survive.