Inserted the white Hi Res square prop at 0,0,-100000 (units set to 1 metre, scale 1000000), added infinite light - i.e. prop was 100KM from camera and it still rendered in Superfly and preview mode ok, Firefly the prop went invisible at 2.6KM or greater. I tried both ray trace and depth map shadows for the lighting but didn't make a difference with the Firefly render.