Apply fix for newer Firefox from adopted-ember-addons#606

Author: mweidner037

addon/mixins/sortable-item.js

@@ -551,7 +551,8 @@ function getBorderSpacing(el) {
   el = $(el);
   if (el === undefined || el === null) return 0;
   let css = el.css('border-spacing'); // '0px 0px'
-  let [horizontal, vertical] = css.split(' ');
+  const [horizontal, initialVertical] = css.split(" ");
+  const vertical = initialVertical === undefined ? horizontal : initialVertical;
 
   return {
     horizontal: parseFloat(horizontal),