However Trump cannot fire the FBI director because he was continuing to investigate Trump and staff members and any illegal collusion with Russian officials.  Flynn was already caught lying about his Russian ties.  If a case can be made, with Comey's testimony, that Trump fired Comey to end/stifle the Trump / Russian investigation, that is an impeachable offense.  Presidents of the US are given incredible power, however like Nixon, the line is drawn when they act like scheming dictators.