Bug 1490366 - Part 3: Select the first item in richlistbox on focus r=dao

Differential Revision: https://phabricator.services.mozilla.com/D8282

--HG--
extra : moz-landing-system : lando

toolkit/content/widgets/richlistbox.xml

@@ -888,6 +888,10 @@
           if (this.getRowCount() > 0) {
             if (this.currentIndex == -1) {
               this.currentIndex = this.getIndexOfFirstVisibleRow();
+              let currentItem = this.getItemAtIndex(this.currentIndex);
+              if (currentItem) {
+                this.selectItem(currentItem);
+              }
             } else {
               this.currentItem._fireEvent("DOMMenuItemActive");
             }