By Alek Arend Ron Rivera made a surprising announcement to begin his press conference this Tuesday morning. The longtime NFL head coach has fired one of his assistant coaches. Rivera announced this morning that he’s fired defensive line coach Sam Mills. He called it a “difference of opinion” between himself and Mills. A surprising coaching staff change in the middle of training camp. “Ron Rivera said he has fired DL coach Sam Mills. Called it a difference of opinion,” said John Keim. “Ron Rivera opens today’s presser by announcing Sam Mills III is no longer part of the coaching staff,” wrote Z…