The most comforting thing about the Trump Administration is that their incalculable malice has, time and again, been surpassed by even greater incompetence.
All signs suggest that Trump and Sessions seriously believed that people would buy the "he's being fired because we're so appalled by his unethical behavior that won us the election" excuse, and that Democrats would praise them and the news media would be indifferent.
It also seems plain that, if this is because there's something in the Russia investigation that Trump or his people didn't want the FBI to find (and I haven't heard any other explanation that's more plausible to explain Trump's motivation here, though "He was just pissed off at Comey and fired him" is entirely within the realm of possibility; it could have just been a simple, petty act by a simple, petty man), then Trump just increased the possibility that the FBI will find it, because the rank and file really aren't going to like this. And if he puts one of his stooges in charge to try and bury the investigation, it'll actually do the opposite and galvanize the lifetime civil servants in the agency.
Maybe. Who the fuck knows, really?
Meanwhile, I can't help noticing that most Congressional Republicans haven't said anything about this. They're in their usual wait-and-see-how-this-plays-out mode.