While Newt has certainly beclowned himself in recent days, I'm beginning to think this article is slander on him, as I cannot find his exact words of "the president cannot obstruct justice".
There are two different ideas here: The president has the right to fire whoever he wants (I agree with this), and the president cannot obstruct justice (I disagree with this). I cannot find Newt's comment about the latter in context. And context is everything in this case.