The firing of James Comey explanation is simply not creditable.
Donald Trump did not even have the decency to face James Comey when he fired him. Unlike his Apprentice program where he fired the contestants, in real life Mr. Trump lacked the courage of his convictions. Perhaps that is because Trump knows that Comey is closing in on the Trump Russia connection. Is there any other reasonable explanation? Politico and The New York Times seem concur with my explanation that I reached within minutes of the announced firing.
[Trump] had grown enraged by the Russia investigation, two advisers said, frustrated by his inability to control the mushrooming narrative around Russia. He repeatedly asked aides why the Russia investigation wouldn’t disappear and demanded they speak out for him. He would sometimes scream at television clips about the probe, one adviser said. [Politico]
Mr. Trump explained the firing by citing Mr. Comey’s handling of the investigation into Hillary Clinton’s use of a private email server, even though the president was widely seen to have benefited politically from that inquiry and had once praised Mr. Comey for his “guts” in his pursuit of Mrs. Clinton during the campaign.
But in his letter to Mr. Comey, released to reporters by the White House, the president betrayed his focus on the continuing inquiry into Russia and his aides. [New York Times]
James Comey was the Director of the FBI since September 4, 2013. Mr. Comey learned of his dismissal while in Los Angeles. Here is the letter that informed Comey of his immediate dismissal.

