As the title states, adjusting the spawn rates for firefly spawns has no effect; additionally, fireflies appear to spawn in some biomes outside of the assigned list. For example, fireflies were spawning in the cherry grove despite the only allowed biomes being the swamp and the mangrove swamp. I also observed that turning the spawn rate for all biome particles to 0 also had no effect.

Fireflies are a little confusing since they actually have two ways of spawning. The first is the biome, which will spawn fireflies all throughout the biome (by default this is limited to swamps). The second is from the vegetation, which will spawn fireflies around grass and flowers in any biome if it isn't too cold, and can be disabled in the Blocks config category. The main difference between them is that the vegetation fireflies will spawn less frequently and lower to the ground than the biome fireflies. I realize this is a confusing setup, but I'm unsure of the best way to make them less confusing since they are two separate features.
If you have any suggestions for solving this issue I would be happy to hear them