Trump said that the former director was fired as Comey was "not able to effectively lead the bureau," but as Comey was in the middle of an investigation into whether the Russian Kremlin had corrupted the US election in order to swing the result into President Trump's favor rather than rival Hilary Clinton, the abrupt and rapid firing -- without a successor in place -- has raised suspicions at to whether Trump fired the director to protect himself and his new position.
Indeed, the firing itself could not have been much harsher, as Comey reportedly found out he had been fired through televisions playing in the background as he spoke to a group of FBI employees in Los Angeles and originally believed the news was no more than a prank.
The testimony is due to start at 10am ET (3pm in the UK), followed by a non-televised closed session around 1pm.
You can watch a broadcast of the testimony on television on many regular and cable channels including CBS, CNN, NBC, PBS, Fox News, and ABC. The UK can also tune in on the BBC News channel.
Comey is expected to reveal details relating to his conversations with the president, as well as whether Trump asked him to end his investigation into alleged Russia ties during the campaign.
The former director will potentially be asked about former National Security Adviser Michael Flynn, who reportedly has turned over at least 600 pages of documents to the Senate related to potential Russian ties, but has invoked his Fifth Amendment rights to ignore a subpoena requiring records of his dealings with Russian officials.
Comey may also be asked questions relating to meetings with the president, whether or not any evidence surfaced during the campaign investigation, and also to comment on any specific directives Trump gave Comey during his time at the FBI.
However, it is not expected that Comey will be required to reveal information related to any other Russia-based investigations -- at least, not during the public televised section of the testimony.
If any new information is disclosed which could be used in criminal investigations, Comey's testimony could become key to prosecutions. However, we simply won't know until the meeting tomorrow.
A tweet sent out by the President on May 12 said, "James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" leading to calls for the release of any such recordings which may relate to Comey's firing.
According to associates of the president, Trump is "spoiling for a fight" and has spent a large portion of his time fuming at "fake news," the media, and coverage of the alleged links between Russia and Trump's electoral campaign.
"The president's going to have a very, very busy day," White House press secretary Sean Spicer told the Washington Post. "I think his focus is going to be on pursuing the agenda and the priorities that he was elected to do."
Weeks afterward, a new director is yet to be found, with sources close to the recruitment process calling the situation "chaotic." Trump himself is leading the interviews but there is reportedly "no clear framework or logic for who was interviewed and why."
Must-have mobile apps to encrypt your texts and calls