It seems pretty obvious why he was fired.
Look, the government backs nearly all climate change research. They love climate change because it gives them a great tool to scare the public into giving up their liberties, and to funnel taxpayer money to their cronies. This guy works for the government and writes a book attacking climate change. The government then fires him. I think it's pretty safe to say why they fired him.
Except that EXXON did its own research during the 1980s, and had come up w/ the same results that scientists now are simply refining.....