The only way Durham can be fired is if he's accused of gross misconduct. Barr made certain of that by appointing him as Special Council.
Merrick Garland can remove Durham the real reason won’t matter if asked to Remove Durham, Garland will come up with the appropriate reason.
Here is the statute:
(d) The Special Counsel may be disciplined or removed from office only by the personal action of the Attorney General. The Attorney General may remove a Special Counsel for misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause, including violation of Departmental policies.
After reading the statute there should be no doubt that Garland could remove Durham. Before any removal of Durham, the case will be made in the media that Durham is a russian stooge, violated DOJ procedures or any number of other reasons.
Here is the pertinent cite ...
(d) The Special Counsel may be disciplined or removed from office only by the personal action of the Attorney General. The Attorney General may remove a Special Counsel for misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause, including violation of Departmental policies. The Attorney General shall inform the Special Counsel in writing of the specific reason for his or her removal.
28 CFR 600.7 - Conduct and accountability.
Close him out with a finding of "improvidently appointed."
(a) An individual named as Special Counsel shall be a lawyer with a reputation for integrity and impartial decisionmaking, and with appropriate experience to ensure both that the investigation will be conducted ably, expeditiously and thoroughly, and that investigative and prosecutorial decisions will be supported by an informed understanding of the criminal law and Department of Justice policies. The Special Counsel shall be selected from outside the United States Government. Special Counsels shall agree that their responsibilities as Special Counsel shall take first precedence in their professional lives, and that it may be necessary to devote their full time to the investigation, depending on its complexity and the stage of the investigation.
28 CFR S: 600.3 - Qualifications of the Special Counsel.