There are plenty of arguments and opinions that say he can’t. Enough that even if he tried it would wind up in the courts, and ultimately the SC.
It’s the AG’s job. And he has recused himself, so it would be Rod Rosenstein’s job, the guy who hired Mueller in the first place.
But it seems to me the majority of people who are speculating that he might, are people who also seem to be dedicated to being critical of everything he does. They are engaging in speculation as if the speculation was itself proof of wrongdoing.
Not that he doesn’t merit plenty of criticism. He brings alot of shit on himself.
But there is clearly an industry centered around making sure that he is always presented in the most unfavorable way possible. And the speculation about firing Mueller is an example of that.