The Fire Warning Snippets for the PMDG 777 are configured to use the SDK variables that are available in the P3D versions of the aircraft.
If you wish to use Fire Warning Snippets in FSX when using the Engine 1 & 2 Fire Control Module with the PMDG 777, change the Snippets on the fire warning indicators to use Fire Warning Engine 1 PMDG 777 FSX and Fire Warning Engine 2 PMDG 777 FSX when the module has been added to a Device Panel.