Rosenstein wasn’t just supportive of Trump firing Comey. Rosenstein and Sessions both agreed on recommending that Comey be fired. Rosenstein wrote the recommendation letter to the President,then he indeed did turn around and hire Mueller once Comey was fired. The entire chain of events starting with Sessions’ promise to recuse when asked during his confirmation hearing, stinks worse than a pile of corpses.
The RR/Sessions recommendation to fire Comey was a Deep State set up from the get go.