Treat any cargo fire as LAND ASAP red and fly with Vmo/Mmo to the nearest airport. You can try to extinguish the cargo fire and solve any other problems on the way to the airport, but first priority is getting to the runway ASAP.
Unless you packed and loaded the cargo yourself, you can never know what it actually contains. If the cause are lithium batteries, nothing will help you but landing as soon as possible and getting as far away from the aircraft as possible.
Next time anybody suggest you that you can easily continue to the destination with the cargo fire extinguished, have them read the final report of UPS 744 crash in Dubai. That should change their opinion on the severity of cargo fire.