Dont fire them - it allows them the process of suing. Instead, remove their clearance for “personal conduct”. No clearance, then no longer able to meet the requirements for the job.
They can still be a dishwasher or door knob polisher ....
FSO’s are commissioned officers just as those in the military and PHS. As such they serve at the discretion of the President and can be relieved at any time without any civil service protections. A simple statement of relief for cause or for no longer being required should do it.
The U.S. diplomatic corps, much of which hails from far-Left Ivy League institutions . . .
I scored in the top 2% in their exam once and was a witness up close and personal to how they screened out anyone who didn't hail from on of their favored institutions other than an extremely token small number.
More likely just a knob polisher ....