Your ignorance is astounding. The FBI director can only be fired for cause ****. The FBI director reports to a Deputy Attorney General who then reports to the US Attorney General. Both of these positions recommend to Trump Comey be fired for cause.
On May 9, 2017, President Trump dismissed Comey after the recommendation of United States Attorney General Jeff Sessions and Deputy Attorney General Rod Rosen Stein. Rosenstein's memorandum to Sessions objected to Comey's conduct in the investigation into Hillary Clinton's emails.