Reduce the "maximum texture resolution" setting in firefly (keep it powers of two - 512/1024/2048/4096) or reduce the texure resolutions of the objects themselfs (by hand) according to the distance to the camera. Remove unneccessary bumpmaps. Don't use displacement if you don't use it. Maybe the material setting of the object makes firefly go crazy (if you use P5 material setting). Try to change that.