You don't see Firefly on AR's because they don't need it to deal with large groups of enemies. It's fairly easy to deal with hordes as long as you have one that's suited for the task. Abyss Defiant, for example, works wonders in the Abyss because of it's high ROF and great stability. I can chainsaw through waves of Thrall without a problem. It's not so great at mid-range, but that's what low ROF, high impact AR's like Suros are for. You say they've ruined it. I've primarily used AR's, and they still work great for me.