MichaelG, the Director of the FBI oversees the agency but does not actively involve himself in investigations.  They are conducted by professional FBI agents and continue regardless of changes in top management.   Thus, Comey’s firing will not stop any investigation.  Watergate was a bit different.  The FBI initially was involved in the investigation, but it was then turned over to an independent special prosecutor, Archibald Cox, who was the principal investigator.  Cox’ investigation led him directly to President Nixon from whom he demanded crucial evidence being held by him.  Nixon refused. Cox refused orders from the White House to cease his demands for the evidence and was fired by the President.   It stopped the special prosecutor’s investigation, but it led to Nixon’s impeachment and subsequent resignation.