Muller has both authority to investigate the reason Comey was fired. FBI director is an at will employee with zero protection.
Furthermore, he cannot go off on fishing expeditions. The parameters of his investigation are set byn the AGs office.
Mueller and Comey are BFFs.
Sadly, you are incorrect. Mueller has free reign via Rosenstein's appointment letter, which includes the authorization to investigate “any matters that arose or may arise”
The very definition of fishing expeditions.