"Mr. Comey’s fate was sealed by his latest testimony... Mr. Trump burned as he watched, convinced that Mr. Comey was grandstanding."

"He was particularly irked when Mr. Comey said he was 'mildly nauseous' to think that his handling of the email case had influenced the election, which Mr. Trump took to demean his own role in history. At that point, Mr. Trump began talking about firing him. He and his aides thought they had an opening because Mr. Comey gave an incorrect account of how Huma Abedin, a top adviser to Mrs. Clinton, transferred emails to her husband’s laptop, an account the F.B.I. later corrected...."

Excellent detail to the story of how Trump fired Comey, in "'Enough Was Enough': How Festering Anger at Comey Ended in His Firing" (NYT).

Also: "... Mr. Comey thought the president was unlikely to get rid of him because that might be interpreted as a conclusion that the F.B.I. director was wrong to announce shortly before the election that he was re-examining the email case, which would call into question the legitimacy of Mr. Trump’s victory." Comey thought wrong, and it says something about how Trump analyzes these things. The election was already called into question. The questioning was continual and unrelenting. Not firing Comey would never stop the questions. Might as well go ahead and fire him, since everything Trump says and does is used against him. Trump must be inured to criticism, and that stupid mistake about Huma created an opportunity.