I think the answer to this question is dependant on how long you plan on housing the two together. Since the smaller of the pbass species commonly kept in aquariums can easily reach 20"-24", eventually it would be large enough to eat a fully grown firemouth. (If you need proof of this, do a search for the video of Trust's pbass eating some 5"-6" jack dempseys.) So if you only plan on keeping the two together while you grow out the pbass, they should be fine together. If you plan on keeping the two together permanently, forget about it. Your firemouth will certainly end up being a nice snack for your pbass.