Rosenstein directing Mueller to end the investigation is not the same as Trump firing Mueller
Neither is Trump suggesting that Rosenstein should do that.
The only thing that comes close to being the same as firing Mueller is if Trump threatened to fire Rosenstein if he did NOT direct Mueller to end the investigation.
Otherwise, there is nothing that Trump has said or tweeted that comes anywhere near being the same as a threat to fire Mueller. Rosenstein’s job is to oversee what Mueller is doing, which includes making a determination over whether he should tell Mueller to wrap things up, and Trump is certainly allowed to express his opinion on that issue, which in no way impedes Rosenstein’s independent oversight over the Mueller investigation, and in no way comes anywhere near being the same as a threat to fire Mueller.