Its faily simple, and this can we done with any mod that is looking to patch the "RG_TemperateGrassland" biome.

Go to the Fireflies2 folder and open the "Patches_RegrowthCoreBiomes" file using a program like Notepad++ or other text editor.

Then simply replace "RG_TemperateGrassland" with "TemperateForest" then save.

This should solve the issue.