Forge has a method level.getRainLevel() that controls how heavy rain is, from 0 to 1. TerraFirmaCraft overrides this method, and makes it return values way outside the 0 to 1 bounds at the start and end of rain, when rain is "fading" in and out. This breaks compatibility with the Pretty Rain mod, and likely others.

This bug also likely breaks the vanilla "fading" in and out behavior, but I haven't been able to confirm that because it's hard to see and rain changes infrequently.

Turns out the bug was with the other mod