If you have an engine fire from a loose fuel connection, it certainly wouldn't hurt to shut off the fuel pump to stop pumping and spraying more fuel under the hood. Probably would still be totaled and not change the conclusion, but I would expect the fire would develop a bit slower if more diesel wasn't being pumped into the fire.

In any case, if I see smoke coming from under the engine compartment, I'll turn the ignition off to depressurize the fuel system as I leave the vehicle.