Well tbh, it was only when I got my royal gramma that caused my firefish to always be out. The gramma just stole the cave and so the firefish decided to stay out all the time, even at night. Then the royal gramma got velvet and died, but the firefish still remained outside, because he just got used to it, and liked it better. So best I can say is actually to get something that would prompt it to come out (I.e. slightly more territorial fish, second firefish). I know they prefer live foods, so if you feed it live brine shrimp and stuff it may have the incentive to go out and hunt. Eventually, I actually got him to follow my finger when I fed him. Unfortunately, I did lose him to what I can figure was a mean emerald crab. :(
Btw speaking of a second fish, anothet firefish could work. you could always do a red firefish to confuse it or some other type of dart fish, that way there is a lessened chance of fighting. I’d avoid one of the same color though.