Since there’s already good cause, his answer is meaningless.
He also said he could not imagine Mueller placing himself in a position that would provide good cause for his firing.
Really now. Considering all of Trump's past treatment of Mueller and the Special Council, and now his hiring a 20+ year personal friend of his as AG, things are becoming pretty clear. Trump is OK with Mueller's investigation. Mueller is valuable to Trump as a foil, a nemesis, a twitter target, a boogie man. He can continue to rail against the great conspiracy. If you want to reel someone in, limit them, or even fire them, you don't hire their long time friend to do it for you.
You think, just maybe, the POTUS could find an AG to oversee Mueller that was not his friend? But no. He picks the guy who has known Mueller for decades. The guy whose daughters weddings Mueller attended. The guy whose wife attends bible study with Mueller's wife. I think Trump is going to miss Mueller when he's gone, or maybe offer him a cabinet post. And I'm only half joking here.