diff --git a/iron-focusables-helper.html b/iron-focusables-helper.html
index 075ff74..e634118 100644
--- a/iron-focusables-helper.html
+++ b/iron-focusables-helper.html
@@ -29,7 +29,7 @@
         // the final array by tabindex.
         var needsSortByTabIndex = this._collectTabbableNodes(node, result);
         if (needsSortByTabIndex) {
-          result.sort(this._tabIndexSort);
+          return this._sortByTabIndex(result);
         }
         return result;
       },
@@ -110,12 +110,6 @@
         }
         var tabIndex = this._normalizedTabIndex(node);
 
-        // TODO(valdrin) uncomment this when delegatesFocus is implemented.
-        // if delegatesFocus and tabindex = -1, should skip its children.
-        // if (tabIndex < 0 && node.delegatesFocus) {
-        //   return false;
-        // }
-
         var needsSortByTabIndex = tabIndex > 0;
         if (tabIndex >= 0) {
           result.push(node);
@@ -153,21 +147,60 @@
       },
 
       /**
-       * Sort by tabindex. Move elements with tabindex = 0 to the end to match
-       * browser behavior, e.g. [0, 3, 1, 2] --> [1, 2, 3, 0]
+       * Sorts an array of nodes by tabindex. Returns a new array.
+       * @param {!Array<Node>} nodes
+       * @return {Array<Node>}
+       * @private
+       */
+      _sortByTabIndex: function(nodes) {
+        // Implement a merge sort as Array.prototype.sort does a non-stable sort
+        // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
+        var len = nodes.length;
+        if (len < 2) {
+          return nodes;
+        }
+        var pivot = Math.ceil(len / 2);
+        var left = this._sortByTabIndex(nodes.slice(0, pivot));
+        var right = this._sortByTabIndex(nodes.slice(pivot));
+        return this._mergeSortByTabIndex(left, right);
+      },
+
+      /**
+       * @param {!Array<Node>} left
+       * @param {!Array<Node>} right
+       * @return {Array<Node>}
+       * @private
+       */
+      _mergeSortByTabIndex: function(left, right) {
+        var result = [];
+        while ((left.length > 0) && (right.length > 0)) {
+          if (this._hasLowerTabOrder(left[0], right[0])) {
+            result.push(right.shift());
+          } else {
+            result.push(left.shift());
+          }
+        }
+
+        return result.concat(left, right);
+      },
+
+      /**
+       * Returns if a node has lower tab order compared to another node (both
+       * nodes are assumed to be focusable and tabbable).
+       * Elements with tabindex = 0 have lower tab order compared to elements
+       * with tabindex > 0.
+       * If both have same tabindex, it returns false.
        * @param {!Node} a
        * @param {!Node} b
-       * @return {Number}
+       * @return {boolean}
        * @private
        */
-      _tabIndexSort: function(a, b) {
-        // Move tabindex = 0 elements to the end of the list.
-        if (a.tabIndex === 0 || b.tabIndex === 0) {
-          // The one with bigger `tabindex` goes on top of the list.
-          return b.tabIndex - a.tabIndex;
-        }
-        // The one with smaller `tabindex` goes on top of the list.
-        return a.tabIndex - b.tabIndex;
+      _hasLowerTabOrder: function(a, b) {
+        // Normalize tabIndexes e.g. in Firefox
+        // `elem.tabIndex` is `-1` for  `<div id="elem" contenteditable>`
+        var ati = Math.max(a.tabIndex, 0);
+        var bti = Math.max(b.tabIndex, 0);
+        return (ati === 0 || bti === 0) ? bti > ati : ati > bti;
       }
     };
   })();
diff --git a/test/iron-focusables-helper.html b/test/iron-focusables-helper.html
index 51f1015..a400b4c 100644
--- a/test/iron-focusables-helper.html
+++ b/test/iron-focusables-helper.html
@@ -46,11 +46,11 @@ <h2>Focusables (no tabindex)</h2>
         <div>
           <input class="focusable1" placeholder="1 (nested)">
         </div>
-        <a href="#" class="focusable2">1</a>
+        <a href="#" class="focusable2">2</a>
         <button disabled> disabled button</button>
         <input disabled tabindex="0" value="disabled input with tabindex">
         <div tabindex="-1">not focusable</div>
-        <div contenteditable class="focusable3">2</div>
+        <div contenteditable class="focusable3">3</div>
       </div>
     </template>
   </test-fixture>
@@ -59,6 +59,12 @@ <h2>Focusables (no tabindex)</h2>
     <template>
       <div>
         <h2>Focusables (with tabindex)</h2>
+        <div tabindex="0" class="focusable7">7</div>
+        <div tabindex="0" class="focusable8">8</div>
+        <div tabindex="0" class="focusable9">9</div>
+        <div tabindex="0" class="focusable10">10</div>
+        <div tabindex="0" class="focusable11">11</div>
+        <div tabindex="0" class="focusable12">12</div>
         <div tabindex="-1">not focusable</div>
         <div tabindex="3" class="focusable3">3</div>
         <div tabindex="4" class="focusable4">4</div>
@@ -130,13 +136,10 @@ <h2>focusables in ShadowDOM</h2>
       test('respects the tabindex order', function() {
         var node = fixture('tabindex');
         var focusableNodes = Polymer.IronFocusablesHelper.getTabbableNodes(node);
-        assert.equal(focusableNodes.length, 6, '6 nodes are focusable');
-        assert.equal(focusableNodes[0], Polymer.dom(node).querySelector('.focusable1'));
-        assert.equal(focusableNodes[1], Polymer.dom(node).querySelector('.focusable2'));
-        assert.equal(focusableNodes[2], Polymer.dom(node).querySelector('.focusable3'));
-        assert.equal(focusableNodes[3], Polymer.dom(node).querySelector('.focusable4'));
-        assert.equal(focusableNodes[4], Polymer.dom(node).querySelector('.focusable5'));
-        assert.equal(focusableNodes[5], Polymer.dom(node).querySelector('.focusable6'));
+        assert.equal(focusableNodes.length, 12, '12 nodes are focusable');
+        for (var i = 0; i < 12; i++) {
+          assert.equal(focusableNodes[i], Polymer.dom(node).querySelector('.focusable' + (i + 1)));
+        }
       });
 
       test('includes tabbable elements in the shadow dom', function() {