The photo was posted just past midnight, after the Fantasy in the Sky NYE fireworks show. The smoke above the Seven Seas Lagoon likely comes from the firing positions located on some « islands » and used during this 360 extravaganza.
I also watched the final minutes of the party in front of the castle, on ResortTV1. The ultimate song played by the DJ was Happily Ever After. What a bold (and fantastic) choice! It caused a very enthusiastic reaction from the crowd!