It is an excellent PvE perk. I adore Firefly. However, the Vendor A.1F19X-RYL has Firefly and better Stability, though the Range is lower and similar stats in all other areas. It's a perfectly fine gun to use stock in PvE. I don't really find Firefly to be that useful in PvP... however, that may be because I play mostly Rumble/Clash where enemies don't group up; maybe in Control it's more useful? The problem I have with the stock model is not that it has Firefly, as that's fine for that perk slot, it's that it lacks Fitted Stock. If you want Firefly in PvP I'd recommend rolling for Fitted Stock/Firefly with any of the red dot sights. It's manageable without any Stability upgrades but it's hard to land all headshots at the firecap.