Not the first!
 

This is maybe a little farfetched, but this passage had offen left me en some kind of doubt...

The obvious thought is that Mandos is foretelling the slaying of Finwë, but could he be talking about Miriel? - wouldn't you consider her 'slain' as well? The very first time I read the SILM my instant thought was Miriel - because I hadn't read the rest of the page... still???

Even if we count Míriel as 'dead' - after all, her fëa has gone to the halls of Mandos - she still could not be labelled as 'slain'.

Apparently, it seemed possible to the Ainur and Eldar that a spirit may leave a body and not return like this; but that one of them might be slain because of the action of another was yet hard to imagine. Fëanor would have considered it murder if the Silmarils had been taken forcefully from him, resulting in his own agony.

What is more, the passing of Míriel was known to everybody, so Mandos' remark could have hardly surprised anyone as it did had they considered it as a comment on Míriel.

I agree, and I also thought that Finwë was already dead at the time that Mandos spoke, so that his words are an example of dramatic irony (Mandos knew what Feanor did not; i.e. that Finwë was dead) -- and a very good example at that -- rather than prophecy.

I'm with Turambar.

After the exchange between Mandos and Feanor, Nienna mourned at the decision of Feanor, and while she was mourning the messengers came from Formenos to tell of the deed Melkor had done. I don't know exactly how far Formenos is from the Ring of Doom, but I always got the impression that the two events (Valar and Feanor; Melkor and Finwe) were taking place simultaneously.

Mandos is said to know all fates within the Great Music, and can only reveal them with Manwe's permission. Miriel was not slain, but chose to depart from physical form as the birth of Feanor was too much for her.