CSS3: The original background image of a fixed position in Firefox does not work with the converted element. Is this an FF error?

Question

Purpose: to create a fixed background position in a transformed element that works in Firefox.

I tried all the solutions on this page (and several others), but no one worked: Fixed focal image binding / fading in chrome combined with css conversion

The things I tried to use are static positions, setting visibility on the back, setting z-index, other materials for attaching the background.

Demo:

https://jsfiddle.net/96u9xqbn/6/

.fixed1 {
   transform: translateZ(0);
}

If you delete the conversion above, it works. But, if you use something like Skrollr or have a different conversion need, then the background is not fixed in firefox.