I don’t think that Trump can fire Mueller. Only his Deputy Attorney General can do that since Sessions recused himself.
Trump can fire anybody in the executive branch of government.
Trump fired Comey on Tuesday, he might as well fire Mueller and all his Dem Operative lawyers who are making salary on the tax payer dime, and getting new laptops with access to EVERYTHING. They need to be fired now, and they need to turn in the laptops. Bye, go home to Mommy.