Ceolacanth, Welcome back.
The question you raised is What happened to the souls of those people who had lived thousand years prior to Jesus and Mohammad's revelation? Are all of them thrown into the fiery pit because they were not Christians or Muslims? Can anyone confirm this?

The first Man on the Earth was Prophet Adam(AS)who was the first messenger of Almighty. And he taught about the Creator and how to Thank HIM. Since then each generations were given a Messenger/Prophet by Almighty and taught them how to live(how to obey/thank The Almighty). So who ever followed the teachings of the time and lived accordingly will be given the reward and who denied will be punished. So don't think that the souls of who lived before Isa(AS) and Mohammed(SAW) will thrown to the fire. The Almighty won't do injustice to any of HIS creations. He is the supreme Judge and He is Omnipotent. So, who did good will be rewarded and who did bad will be punished(Unless he repent sincerely) irrespective of his religion/race/colour.