Feanor nods as he listens to recollection of his adventures. He openly smiles when Curufin declares his continued defiance of whatever Mandos, or any other Valar, would want of him. He's glad that his son resisted. The Halls are a sad place, they'd have dampened even his enthusiasm and this would simply be a crime.

How he missed the sound of that laughter...

It is the last sentence of that tales of tales that hits him with the weight of a mountain. The very fact that it is exactly Void, even if not The Void, they are traveling through right now made Feanor assume his own curse is still well in effect. What if it's not? What if he's free?
It takes a couple of minutes before the worst of the shock passes and he's able to say anything. Even if it's not the most coherent question.

"You are?! How?!"