--- a/toolkit/modules/SelectContentHelper.jsm
+++ b/toolkit/modules/SelectContentHelper.jsm
@@ -108,16 +108,17 @@ function buildOptionListForChildren(node
       let textContent =
         tagName == 'OPTGROUP' ? child.getAttribute("label")
                               : child.text;
       if (textContent == null) {
         textContent = "";
       }
 
       let info = {
+        index: child.index,
         tagName: tagName,
         textContent: textContent,
         disabled: child.disabled,
         display: child.style.display,
         // We need to do this for every option element as each one can have
         // an individual style set for direction
         textDirection: getComputedDirection(child),
         tooltip: child.title,

--- a/toolkit/modules/SelectParentHelper.jsm
+++ b/toolkit/modules/SelectParentHelper.jsm
@@ -57,19 +57,18 @@ this.SelectParentHelper = {
 
   _unregisterListeners: function(popup) {
     popup.removeEventListener("command", this);
     popup.removeEventListener("popuphidden", this);
   },
 
 };
 
-function populateChildren(menulist, options, selectedIndex, zoom, startIndex = 0,
+function populateChildren(menulist, options, selectedIndex, zoom,
                           isInGroup = false, isGroupDisabled = false, adjustedTextSize = -1) {
-  let index = startIndex;
   let element = menulist.menupopup;
 
   // -1 just means we haven't calculated it yet. When we recurse through this function
   // we will pass in adjustedTextSize to save on recalculations.
   if (adjustedTextSize == -1) {
     let win = element.ownerDocument.defaultView;
 
     // Grab the computed text size and multiply it by the remote browser's fullZoom to ensure
@@ -93,30 +92,28 @@ function populateChildren(menulist, opti
 
     // A disabled optgroup disables all of its child options.
     let isDisabled = isGroupDisabled || option.disabled;
     if (isDisabled) {
       item.setAttribute("disabled", "true");
     }
 
     if (isOptGroup) {
-      index = populateChildren(menulist, option.children, selectedIndex, zoom,
-                               index, true, isDisabled, adjustedTextSize);
+      populateChildren(menulist, option.children, selectedIndex, zoom,
+                       true, isDisabled, adjustedTextSize);
     } else {
-      if (index == selectedIndex) {
+      if (option.index == selectedIndex) {
         // We expect the parent element of the popup to be a <xul:menulist> that
         // has the popuponly attribute set to "true". This is necessary in order
         // for a <xul:menupopup> to act like a proper <html:select> dropdown, as
         // the <xul:menulist> does things like remember state and set the
         // _moz-menuactive attribute on the selected <xul:menuitem>.
         menulist.selectedItem = item;
       }
 
-      item.setAttribute("value", index++);
+      item.setAttribute("value", option.index);
 
       if (isInGroup) {
         item.classList.add("contentSelectDropdown-ingroup")
       }
     }
   }
-
-  return index;
 }