General advice is to add the firefish before other fish like clowns so that it doesn't impede on their territory, not sure if that has any influence here though as you see no aggression. My firefish has a dart hole under the live rock where it sleeps at night so I know it's safe. When I bought it I could see its fins were in bad shape from bullying, now great a few weeks later after some time on its own, and even now with a small clown pair. Maybe there's some aggression at night? A change in rock layout can reset territories. Otherwise I'd maybe try getting a closer picture and see if anyone can see signs of disease etc. Also check your water parameters and flow.