The longer Trump waits, the worse it looks if he ever actually does pull the trigger. But then, tweeting about how much he'd like to fire Mueller is apparently also providing evidence in an obstruction case. Although, he's been extremely patient, letting Mueller run with his investigation, but, I'm sure to him, its run its course, time to wrap things up. Unfortunately, Sessions, Rosenstein, and Mueller have all succumbed to the deep state, so it is with great sadness (particularly for Jeff) that I have to let them go.
The above will play just fine with his base, and that's all he cares about.. but there will be some actual consequences for firing all three men. For one thing, there are likely to be other resignations involved throughout the Justice Department and maybe the FBI as well, and potentially among his inner circle as well. (There are rumors that Don McGahn, for one, threatened to resign when he was asked to stop the Mueller investigation shortly after it began.) His true believers will probably cheer the backs of all these people, but even some people who generally support him will think this looks bad.
There will also be pushback from Senate Republicans for firing Sessions, in particular. He was well liked there and Chuck Grassley (who would be in charge of the committee hearings on any replacement) has made it clear he isn't interested in seeing Sessions go anywhere. Plus, taking Sessions out of the Senate resulted in losing possibly the most reliable Republican seat imaginable, so it would be doubly insulting to the Senate Rs to see that happen without even having Sessions as AG where he's doing a bunch of stuff that Trump really would approve of even if he probably isn't aware of it.
This isn't to say that Trump won't actually do it at some point. I think the chances are pretty good that he might. But he's probably hearing from a lot of people that he really shouldn't.
But, hey...when a guy under criminal investigation says he's innocent, that should be good enough for everybody, right?