That would be Comey, not Mueller. And I agree that Trump had all the authority in the world to fire Comey.
Then why did he enlist the help of Sessions and Rosenstein?
HE was the one who got those two involved in that whole charade, and now he's running around humiliating Sessions in public and pretending he doesn't know who Rosenstein is? LOL.
According to Breitbart, it was Sessions who pushed for Rosenstein as Deputy:
‘Sessions was reported to have played a significant role in the selection of Rosenstein by President Donald Trump for this important position. Speaking to a business group Monday, Sessions emphasized the centrality of the Deputy AG position to the effective functioning of the Justice Department and expressed his excitement at Rosensteins imminent arrival at DOJ. Thats the central person in your team, Sessions said, Virtually all departments report to the deputy, who manages the department. Its a powerful position.’