You're actually wrong, I believe.
History says I am right.
That was a major part of the Saturday Night Massacre. Nixon couldn't fire the special prosecutor directly
Your premise is patently false. The ability to fire Cox was never an issue. It is simply a matter of protocol. The CEO of Wal-mart would not fire a cashier at a store in Aberdeen, South Dakota who insulted a customer. Instead, he would give the order to the retail VP who would issue an order to the Regional manager who would issue an order to the store manager. If anyone in that chain refused to implement that order, then the CEO would have a much bigger problem on his hands that needed to be resolved first. Thus the resignations of Richardson and Ruckelshaus, and the promotion of Bork.
Here is a Justice Department organizational chart from 2003:
Any order to fire a US Marshal would come from the AG to the Deputy AG to the Director of the US Marshall service to the Deputy Director. If the President issues an order, it gets passed down the chain of command. In no way does it mean that the President can't fire that person. Executive branch employees work for the head of the Executive Branch.
It was only a few weeks ago that President Trump fired the Deputy Attorney General.
In 2007, the Bush White House ordered the firings of 7 US attorneys. The special prosecutor that was appointed to investigate concluded that there was nothing illegal about the firings, even without the cooperation of White House counsel.