Honestly, we know enough. From NFL.com:
Reports of angst between Mills and the defensive line surfaced this offseason after the group underperformed in 2021. But Rivera noted that the decision to fire Mills stemmed from more recent philosophical differences.
"It's about this offseason and training camp," Rivera said.
His firing probably stems from underperformance, and Rivera didn't like the way things were progressing during the offseason and camp. It sounds like it was a long time coming. Rivera was reticent to just fire his friend so easily, so he gave him a chance, but things didn't work out.