At no stage did the parent post state there are no good engineering managers at Mozilla. They just said that adjusting bug priorities is the work of a good engineering manager. There's a world of difference.
If you want to complain about knee-jerk overreactions, I think you might want to look in the mirror first.
So we can come to the obvious conclusion about Mozilla, then? No good "engineering" managers? Miss one reprioritization and you're out! This is what sane people think?