The problem is that you have 2 fish in there and the JD wants the territory. Put 5 or so bigger cichlids than the JD in there and it will stop. Then, as they grow, remove the problem fish or the ones that are getting too big or upsize the tank and keep them all.
As long as you have adequate filtration, Cubans, Convicts, Neets, Carpintis, female Cyanno or something along those lines that is tenacious, will stand its ground and wont outgrow the tank will work. Firemouths arent, IME, that aggressive (neither are any JDs Ive had either). Just keep in mind my suggestion isnt for the tank for life. You will eventually need to upgrade or remove several of the fish as they get bigger.
If you need help with all this, go see Adam @ Fish World in Raleigh. He can solve the problem for you with ease...and if they dont have any of the aforementioned fish I suggested, PM me. I have several between 2-5 inches that Id sell you...or maybe give you.