DUBAI, United Arab Emirates — An Emirates flight from India with 300 people on board crash landed at Dubai’s main airport Wednesday, sending black smoke billowing into the air and halting all flights at the Middle East’s busiest airport. A firefighter died while responding to the blaze, but none of the passengers or crew on board were killed. The accident was the most serious ever for Emirates, which has grown at a breakneck pace over the last three decades and turned its hometown of Dubai into a major long-haul international air hub.