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 resigned from the Broncos following the 2014-15 season. A technicality, I’ll give you that.
John Fox was absolutely fired.
He “mutually parted ways” on January 12, 2015. He was definitely their HC for the '14-'15 season.