Via the New York Times:

Mr. Comey, who is three years into a 10-year term at the helm of the F.B.I., learned from news reports that he had been fired while addressing bureau employees in Los Angeles. While Mr. Comey spoke, television screens in the background began flashing the news. Shortly after, a letter was delivered to F.B.I. Headquarters in Washington.
…
The president has the authority to fire the F.B.I. director for any reason. Officials at the F.B.I. said they learned through news reports of Mr. Comey’s dismissal, which Mr. Trump described as effective immediately.

That’s not a good way to find out you’ve been fired. And apparently Comey thought it was a joke:

Mr. Comey was addressing a group of F.B.I. employees in Los Angeles when a television in the background flashed the news that he had been fired.

In response, Mr. Comey laughed, saying he thought it was a fairly funny prank.

Then his staff started scurrying around in the background and told Mr. Comey that he should step into a nearby office.

Mr. Comey stopped addressing the group. He proceeded to shake hands with the employees he had been speaking to. Then he stepped into a side office, where he confirmed that he had been fired. At that point, he had not heard from the White House.

Douglas MacArthur found out he’d been relieved of command via a commercial radio broadcast while he was at lunch, but Truman had tried to inform MacArthur privately via personal messenger who was delayed in transit. The decision to fire Comey in a fairly humiliating way seems to be deliberate and calculating. I hope so. If Comey was fired in this way to send a message throughout the government that he has no fear of firing people, I’m all for it. I fear, however, that this was a petty act of revenge designed to do nothing more than exact some punishment for perceived wrongs.