diff --git a/accessible/public/nsIAccessible.idl b/accessible/public/nsIAccessible.idl
index 281e85bb2c4..84c3d77f308 100644
--- a/accessible/public/nsIAccessible.idl
+++ b/accessible/public/nsIAccessible.idl
@@ -54,7 +54,7 @@
  *
  * @status UNDER_REVIEW
  */
-[scriptable, uuid(B26FBE47-9A5F-42a1-822B-082461AE4D6D)]
+[scriptable, uuid(A84FCE8C-B127-4528-84F1-A022F5B040DA)]
 interface nsIAccessible : nsISupports
 {
   /**
@@ -87,6 +87,12 @@ interface nsIAccessible : nsISupports
    */
   readonly attribute long childCount;
 
+  /**
+   * The 0-based index of this accessible in its parent's list of children,
+   * or -1 if this accessible does not have a parent.
+   */
+  readonly attribute long indexInParent;
+
   /**
    * Accessible name -- the main text equivalent for this node
    */
diff --git a/accessible/src/atk/nsAccessibleWrap.cpp b/accessible/src/atk/nsAccessibleWrap.cpp
index 8d83579636c..3993732ab2c 100644
--- a/accessible/src/atk/nsAccessibleWrap.cpp
+++ b/accessible/src/atk/nsAccessibleWrap.cpp
@@ -858,32 +858,8 @@ getIndexInParentCB(AtkObject *aAtkObj)
     nsAccessibleWrap *accWrap =
         NS_REINTERPRET_CAST(MaiAtkObject*, aAtkObj)->accWrap;
 
-    // we use accId to decide two accessible objects are the same.
-    void *accId = nsnull;
-    NS_ENSURE_SUCCESS(accWrap->GetUniqueID(&accId), -1);
-
-    nsCOMPtr<nsIAccessible> accParent;
-    nsresult rv = accWrap->GetParent(getter_AddRefs(accParent));
-    if (NS_FAILED(rv) || !accParent)
-        return -1;
-
-    nsCOMPtr<nsIAccessible> accChild;
-    nsCOMPtr<nsIAccessible> accTmpChild;
-    accParent->GetFirstChild(getter_AddRefs(accChild));
-
     PRInt32 currentIndex = -1;
-    void *currentAccId = nsnull;
-    while (accChild) {
-        ++currentIndex;
-        nsCOMPtr<nsIAccessNode> accNode(do_QueryInterface(accChild));
-        if (accNode) {
-            accNode->GetUniqueID(&currentAccId);
-            if (currentAccId == accId)
-                break;
-        }
-        accChild->GetNextSibling(getter_AddRefs(accTmpChild));
-        accChild = accTmpChild;
-    }
+    accWrap->GetIndexInParent(&currentIndex);
     return currentIndex;
 }
 
diff --git a/accessible/src/base/nsAccessible.cpp b/accessible/src/base/nsAccessible.cpp
index 99f1c5b6496..278171ed340 100644
--- a/accessible/src/base/nsAccessible.cpp
+++ b/accessible/src/base/nsAccessible.cpp
@@ -428,6 +428,36 @@ NS_IMETHODIMP nsAccessible::GetChildCount(PRInt32 *aAccChildCount)
   return NS_OK;  
 }
 
+/* readonly attribute long indexInParent; */
+NS_IMETHODIMP nsAccessible::GetIndexInParent(PRInt32 *aIndexInParent)
+{
+  *aIndexInParent = -1;
+  if (!mParent || !mWeakShell) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIAccessible> sibling;
+  mParent->GetFirstChild(getter_AddRefs(sibling));
+  if (!sibling) {
+    return NS_ERROR_FAILURE;
+  }
+
+  *aIndexInParent = 0;
+  while (sibling != this) {
+    NS_ASSERTION(sibling, "Never ran into the same child that we started from");
+
+    if (!sibling)
+      return NS_ERROR_FAILURE;
+
+    ++*aIndexInParent;
+    nsCOMPtr<nsIAccessible> tempAccessible;
+    sibling->GetNextSibling(getter_AddRefs(tempAccessible));
+    sibling = tempAccessible;
+  }
+
+  return NS_OK;
+}
+
 nsresult nsAccessible::GetTranslatedString(const nsAString& aKey, nsAString& aStringOut)
 {
   nsXPIDLString xsValue;