Latest report is that 45 was po'd watching Comey testify; he was apparently enraged ('white hot' was the description used) by the comment that Comey was mildly nauseous that he might have affected the outcome of  election.  This comment could be read two ways,  that he affected the outcome of an election, and the other, that 45 got elected and that's what made him nauseous.  45 apparently took the latter meaning.  He was also angry that Comey was focusing too much on the Russia connection and not the leaks (latter more important that a foreign entity interfering in U.S. democratic process apparently).   

As the saying goes, if something doesnt make sense its probably untrue.  Another is the simplest explanation is the truth.  It seems more likely that the firing was a product of his rage.  Understandable given his personality but dangerous if the perception is that he is interfering with an investigation, which brought Nixon down.  It does not help that he refuses to release taxes.