Mate of mine departed just ahead of the UPS. Said that the UPS reported "Main Deck Fire indication" and requested the closest airport from Bahrain Control. They suggested Doha which was 100 miles. After a brief pause the UPS crew requested "Why not back to where we came from". They said OK and offered a vector to the 10 mile final 12L. After a minute the UPS requested "Emergency descent due fire". They then informed ATC that the smoke was obstructing their vision and that they could not see anything. After that the comms were with DXB on 121.5 where they were cleared to descend to 9000', which was changed to descend at your discretion. The rest were bits on 121.5.