What does Mandos mean when he says 'Not the First'?

[Lord of Angmar]

Mandos has forboding and often foresees and portends things, but it is not his place to act on this forboding. Yes, he was referring to Finwe's death at Formenos, and some also have said that he meant the death of Miriel, mother of Feanor. I think the first is more likely, because a) it sets up the sack of Formenos and b) Mandos often foretells but it would make no sense in the context if he brought up the obvious (the fact that Miriel was dead).