They are civil service, so they can’t be easily dismissed. Their positions are not political appointments (only the Director is within the FBI).
Let’s hope the proper investigations are being conducted and that McCabe, Baker, Strzok, Page, et al will get fried in every way possible, both within civil service procedures and with relevant criminal prosecutions.
It may be past time to revamp our whole approach to “civil service” which makes it much too difficult to deal with malfeasance, dead weight, and other bureaucratic problems. But that takes Congressional legislation, not simply executive fiat.
But they can be demoted, barred from the building and have their security clearances "temporarily" revoked by the President.
That needs to be done to send the message to the next layer of FBI management, that political activity within the bureau will be dealt with harshly. A lost pension will send just that kind of message.
From what I’ve read, the President can directly fire positions that are “appointed” by the President, of which there are many in the Executive Branch. McCabe’s just doesn’t happen to be one of them.
However I’d be willing to bet that’s just some sort of departmental policy that Trump could overrule if he really tried. Might take more than 90 days though knowing the government.