Currently the two most winningest head coaches ever fired in NFL history are:
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Marty Schottenheimer from the 2006 Chargers. Career 47-33, 0.588
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Tony Dungy from the 2001 Buccaneers. Career 54-36, 0.600
Sean McDermott is now career 68-41, 0.624, including being 6-6 at the moment.
Firing him now would set the NFL record for most winningest HC ever fired.
If he still has the locker room at the end of the season then keep him around. I think that will be dependent on how they finish the year if the players are still backing him. If he stays they need to hire an actual Defensive Coordinator and Assistant Head Coach, preferably someone outside of the McDermott circle of coaches.
If the Jets were to fire Saleh, I would hire him immediately as DC.