fixRespimg = function(img) {
				var source, sizes;
				var picture = img.parentNode;

				if (picture.nodeName.toUpperCase() === "PICTURE") {
					source = dummySrc.cloneNode();

					picture.insertBefore(source, picture.firstElementChild);
					setTimeout(function() {
						picture.removeChild(source);
					});
				} else if (!img._pfLastSize || img.offsetWidth > img._pfLastSize) {
					img._pfLastSize = img.offsetWidth;
					sizes = img.sizes;
					img.sizes += ",100vw";
					setTimeout(function() {
						img.sizes = sizes;
					});
				}
			}
FF's first picture implementation is static and does not react to viewport changes, this tiny script fixes this.