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.
John Fox was absolutely fired.
He “mutually parted ways” on January 12, 2015. He was definitely their HC for the '14-'15 season.