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You are probably better off using AO on individual materials anyway. There is a great product in the store called "Occlusion Master", which is a Python script which lets you set AO on an object by material, or you can do it by hand in the material room. You don't need the overhead of having Firefly calculate the AO on items which won't benefit.

I agree, AO on the figures instead of the lights is a much better solution and can still reduce render times,  since you don't really need it on everything, such as hair. Occlusion Master is a great tool and very easy to use.