The thing is that all this discussion of aircraft design dropping the engineer's panel is superfluous to my mind. Even if the aircraft is a 2-flight crew design, nothing is stopping cargo operators from carrying a third crew member - say a relief pilot (of any rank) on the jumpseat - and training all their flight crew in firefighting. The only question there is cost, and with the margins in cargo operations being significantly tighter than thier pax equivalents it would be a big question.
You cannot generalize about cargo margins. Some cargo carriers -- including UPS and FedEx -- are (and have been) wildly profitable, even while many passenger carriers have suffered huge losses.
More to the point would be whether procedures would be developed to actually use a 3rd crewmember as a firefighter. While FEs on our 747 Classics are allowed to go to the cargo deck to check out smoke, etc, our 744s do not have the proper O2 masks to do that, so a 3rd pilot would make no difference without a significant change in policy, training, and equipment.