Assuming that almost no one is using the firefly render in Poser 13, I tried all the above settings but it will always make a square background no matter if autofit is checked or not. The scene will certainly fall out of the background. If you try loading a new background replace it through the interface it will keep the first one, you can only change it in the material room in order to work in firefly.

So in theory all these auto features and buttons are useless if for firefly the background aspect needs to be done manually ? This would mean that the better choice is to render firefly in a Previous poser version 10 11 or 12 saving allot of trouble.

I hope that it does not also affect the texture setup on a model in firefly when rendering if the autofit is not working as it should making Images 1:1 square. Will have to test it on a square plane to see if it is only happening on the background.