He doesn't have to fire Mueller in order to get this monster back in the box. Congress often demands testimony based on news reports. That's a concession that the report may be false. Trump's tem, and acrually Rosenstein should do this on his own without prompting, and Congress has an interest too, that DoJ follow it's own regulations, all of these people have a right to demand to see the scope of Mueller's investigation. Report are that Mueller is looking at real estate transactions from 2008 and 2013. How can those conceivably be related to an election campaign that didn;t exist before 2015? And how can those transaction possibly be an attempt to influence the outcome of an election, or work to the benefit of the Trump campaign, e.g., illegal money in campaign coffers.
He also gave an even bigger clue when the Department of Justice issued that strange press release (with Rosenstein listed as the official who authorized its release) back in mid-June that gave the following warning:
Americans should exercise caution before accepting as true any stories attributed to anonymous 'officials,' particularly when they do not identify the country -- let alone the branch of government -- with which the alleged sources supposedly are affiliated.
I immediately saw that as a message that was intended to tell the American public that most of the media reports they were hearing about the Mueller investigation were complete bullsh!t.