[Nils]

In short, Tolkien believed that Elves were to be immortal, but because of Melkor's corruption of Middle-earth they could die. When they died, Elven spirits were to go to The Halls of Mandos where they were to purge themselves of any evil that their corrupted bodies added to their souls in life. It is very similar to the Catholic concept of purgatory.

Once the evil has been purged, the Valar were to create new bodies for the Elven souls to enter so that they may live once again.

Feanor was so screwed up and hateful that I don't think he was ever able to purge the evil from his soul.