Dont know how long you have had them, but when juviniles of the same gender mature, or adults of the same gender become comfortable in the tank, they will fight. Often 'till one dies. Might consider seperateing them.
I have Three Firefish in my tank and they all get along fine (100 gallons). Have you ever seen any aggression between the firefish? What else do you have in the tank? Is the fin "broken" or just gone (bitten off) ?