The Daily Beast is not my preferred source, but this is one of the best articles about Comey's firing I have come across thus far:
The Comey firing comes two days before he was scheduled to appear before the Senate Intelligence Committee in an open hearing, to testify about “worldwide threats.” Questions about the FBI’s investigation into possible Trump-Russia ties would have likely surfaced.
... But Rosenstein and his boss, Jeff Sessions, seem to have made their determination about Comey’s handling of the Clinton investigation before the department’s own inspector general apparently finished investigating Comey for the same matter. Inspector General Michael Horowitz said in January his office was investigating Comey’s handling of the Clinton investigation, including the July 2016 news conference that Rosenstein cites as the primary reason he was fired.
Related, Anderson Cooper won't let Kellyanne Conwa, and by extension Trumpsterfire, off the hook with their nonsensical explanation for the firing. It is really worth a watch as he contrasts Candidate Trump with President Trump.