When I got my tank, there were 2 firefish and 1 died before transportation. The other one became very, very skittish.... he was the only fish in the tank. He would come out if no one was around or if you were very still in front of the tank. If you turned your head to watch him, and you were within 5 feet of the tank, he darted back into the rocks. If you approached the tank, he darted into the rocks. It took him a few minutes to come out after you fed him.
Once I added another 2 fish, he was much better. Much less skittish, and stayed out of the rocks more. Now he comes out within a few seconds, as soon as you close the lid after feeding and step back. If he's out, he stays as long as you approach the tank slowly. I think he needed other fish in the tank to feel safe out and about, kind of like discus need dither fish so they don't stay in hiding all the time. Actually, from the little I observed before I bought the tank, when there were still both firefish in it, they acted just as skittish with two in there as the one did on his own... from my experience they aren't too brave, and you don't see much of them if they are alone in a tank. And a 5g isn't big enough for 2 fish.
I would highly recommend a neon goby. They're tank bred, so they're much less skittish and eat whatever (so does the firefish). Mine will cling to your finger and move all over it when you hold food in the water. They stay small enough for a 5g and are pretty fish. Plus, he's out all the time, even most of the time when the lights are out.