I hope he doesn't fire him just so we get to see Mueller suck his own tongue out his starfish when he has to come out and tell everyone Annoying Orange is clean. It's gonna be like gargling is own stuff and piss having to say those words to the public. Glorious.
He is likely just waiting around until the dems retake the house/senate. Then he's going to close the investigation, call it a success, and levy some incredibly petty, extremely mundane charges utterly unrelated to Russia against Annoying Orange. Then the dems will take that and run an impeachment attempt with it.
If Annoying Orange fires him, the dems will simply use that as an excuse.
The only ways Annoying Orange gets out of this setup is if the Repubs maintain control and destroy any hope of the investigation to simply stall, OR an investigation is launched into the corruption of the Russia investigation.
At least thats how I see this going.