VMO/MMO Descent
There are probably multiple reasons for descending at VMO-MMO with a main deck cargo fire. The fact that you are on fire and want to land ASAP is a valid one. I think the core reason has to do with the fundamental philosophy behind how main deck fires are dealt with. Absent fire suppression agents, the fire-fighting method is based on oxygen starvation. The goal is to stay at an altitude where there is not enough oxygen to feed the fire until the last possible second, minimize time at lower (oxygen rich) altitudes, and get the airplane to a spot where a fire department can take over and fight the fire. The steepest available descent from that altitude is at VMO/MMO. Using that speed minimizes the time at lower altitudes which could feed the fire and allow it to get out of hand. Note that this will not help you if the type of fire is one which manufactures its own oxygen.