What does the author mean by “the permanent tenure of judicial offices”? Does he support or oppose this idea?

This would mean that when a judge is appointed, he is appointed for life. Yes, beyond the incoherent babble of this stuff I think he agrees with it,

"If, then, the courts of justice are to be considered as the bulwarks of a limited Constitution against legislative encroachments, this consideration will afford a strong argument for the permanent tenure of judicial offices, since nothing will contribute so much as this to that independent spirit in the judges which must be essential to the faithful performance of so arduous a duty."