When could that happen?
Congress is on Recess every time there is a Holiday and often when there is no Holiday. Usually they take a week off and often longer. Labor Day is Tuesday, May 1st. I suspect they will be in Recess all that week. During that period, Trump can make Recess appointments that don’t require Senate Confirmation. Those appointments have a limited term. John Bolton served as UN Ambassador under a recess appointment for a year under the Bush Administration. If Trump chose to fire Sessions, Rosenstein or anybody else before that Recess, he could then fill the vacancies as Recess Appointments without Congressional Approval. During their time in those positions, they could swing big axes.