It has been a long time since I looked at the book. He mentioned both in the show. Don even says something like, "Well you know, there is some controversy surrounding those documents. I would have to listen to it again, but I think Joe mentions fiber optics and something else and implies or says outright they came from somewhere else. Joe's voice would drop out some and I missed some of what he said myself, trying to listen to it in the car.
I think you somehow misportray his viewpoint. Well, what you say about him didn't fit the feeling Firmage left me in this interview as an individual who would give some credit to some debatable elements but also expressing some doubts and not standing entirely on one side. So I went again to that show, and indeed Firmage didn't mention fiber optics coming from somewhere else nor did he say he believes the MJ12 to be entirely authentic...Actually, he said that he's more inclined to believe that some core in the MJ12 documents has some truth, as well as in the the Corso's claims. These positions may have some logic if one supposes the Roswell incident to be a ufo crash, which Firmage does, while he also said he was not absolutely sure. I haven't read his book (just found it recently on internet) so there maybe some materials from him I'm not aware of, but from that show Firmage didn't give absolute credits to these elements (MJ12, reversed ET technology).