18:1505 says this;
Whoever corruptly, or by threats or force, or by any threatening letter or communication influences, obstructs, or impedes or endeavors to influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States, or the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress—
firing anybody with the intent to influence an investigation is a crime. There needs to be no crime discovered for the charge under 18 1505 to be valid. It is the act of improperly attempting to influence the investigation that is a crime all its own.
While the president can fire whomever he chooses (although that in itself isn’t an accurate statement as I believe many he claimed to be able to fire were not able to be fired by chump. As it was, chump was not empowered to fire mueller), if the action is with the intent to Influence an investigation, that is a crime.
Additionally, The floating of a possible pardon for a person who is under criminal indictment to attempt to cause them to be reticent in their statements violates 18 1510