Currently the two most winningest head coaches ever fired in NFL history are:

2. Marty Schottenheimer from the 2006 Chargers. Career 47-33, 0.588

3. 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.