It probably depends on when the fireworks are on the night you are at Narcoossee's. If they are around 9:00 pm, then you will probably still be eating when they start. Generally, I've read that Signature meals take about 2 to 2.5 hours (it took us about 2.5 hours for appetizers, entree, and dessert).
They dim the lights and pipe in the music. When you are seated, if you can see the castle, then you'll be able to see the fireworks (assuming no one is in the way outside).
We were at Narcoossee's during a cold spell in December, so no one was outside on veranda. As a result, the view from inside was great. However, I have read that during better weather, people go outside and end up blocking the view of those that remain inside.