Yes, you can go out on the observation decks to watch the fireworks. They even have speakers outside to play the music. Last year, my wife and I went up, got drinks from the bar, and took them outside to see the show. About 15 people watched with us. No problem. This year, when we got out of the elevator, there was a server meeting people and telling them they could see the fireworks by going down the hall behind the elevators ( to your left as you step out. ) DO NOT LISTEN TO HIM. He is sending you to the OTHER observation deck. There are two. This one does not have as good a view as the one closest to the restaurant. Not thinking, I went outside and realized my mistake. I returned to the elevators and barged past without making eye contact. He never said a word. We went out to the first observation deck, and saw the show from there. We had my niece and nephew with us, so no drinks this year. By the way, both decks were very crowded, about 30-40 people on each one.
My point to this rather long tale is this...get there early. This has obviously gotten more popular, hence the server trying to keep everyone out of the restaurant. Go early and sit at the bar, get a drink, and wander out and catch the best view anywhere in the MK. As you get off the elevator, turn right, go to the corner, turn right, keep going and you'll see the doors to the observation decks.
Another great thing is that right around the end of the fireworks, the water parade pulls up and does it's show in front of the Contemporary docks. It's kind of corny, but fun to watch. It's sort of like MSEP on water, on a much smaller scale.