He can be terminated for cause. What constitutes cause is a political question the courts won’t touch. So yes he can be fired.
Trump can terminate him for cause, the cause being he interfered in the election by being mean to Hillary!!
How awesome would that be? Hillary has been out in public blaming Comey for her loss. It's not like she can take that back. Trump can agree with her, and say that it's unacceptable for a law enforcement figure to insert himself so prominently into a political race, especially the "sacred" race for the Presidency.
Do it!
Except the courts are all stacked with Obama appointed judges.
No cause is needed, none. He can be fired at will and has no protections. He has a max limit of 10 years.
How about simply He can be terminated.