Square falloff sounds great for the firefly's. You can also try and range the falloff, and use a gradient to control the intensity. That way, you can limit the light's range to something suitable for the scene. Also any light outside the radius will be ignored by the render engine, which means a speed-up at render time compared to non-ranged lights.

You could also try adding a sphere round the light, and give it a fuzzy texture, set to be additive, to make a subtle halo around the visible lights.

Another way to make the fireflies themselves visible would be to make a firefly texture in photoshop or something similar, and put it on 2d planes. If the texture fades to absolute black, you can put the texture to additive, just like with the above suggested halo sphere.

Or you could go for a hybrid approach, and add the halo's in photoshop as postwork.

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