I’m referencing criminal charges for obstruction of justice, if he were to fire either Rosenstein or Mueller. Even though I do not believe a sitting President can be charged or indicted while in office, they can once they are out of office, either via impeachment, or just when their time comes up. Firing either of those men, or Sessions in order to have a new AG who then ends the Mueller probe, would make for a much, much easier case to prosecute him on obstruction.
I believe he fears criminal charges more than impeachment, which is why he has not moved to fire these folks at the DOJ. Not because he is concerned with the optics of it. Like you, I bet he is chomping at the bit to do just that, but is afraid of the criminal penalties that could be levied against him. I believe this because of his statement about Mueller looking at his finances would be considered him crossing a red line. Likely because there are criminal misdeeds in Trump’s finances. He has been heavily involved in New York real estate for decades, and there is no convincing me that every single financial transaction he has ever made has been above board. He knows it too, which is why he warned against looking into his finances.