The background image schould be stretched to fit the viewport completely without cropping any parts of the image.
In earlier versions of Firefox this worked fine with
background-repeat: no-repeat; background-size: 100vw 100vh;Currently instead of independently scaling the width and height of the image to the viewports dimensions, the aspect ratio of the image is maintained leaving unwanted and ugly bars either on top an bottom or on the sides of the viewport depending on the aspect ratio of the viewport.