Yeah, I switched from it when I noticed the inconsistency of the Firefly activating, even when getting crits. I'm guessing the crit may not have been enough to kill them (though I'm calling shenanigans on that, as it will even happen on low level Dregs which should immediately die after the initial precision hit), or the damage is applied so quickly that the explosive still causes damage after the precision hit and doesn't set off the Firefly.