--- a/xpfe/appshell/nsXULWindow.cpp
+++ b/xpfe/appshell/nsXULWindow.cpp
@@ -1631,65 +1631,64 @@ NS_IMETHODIMP nsXULWindow::SavePersisten
   nsCOMPtr<nsIBaseWindow> parent(do_QueryReferent(mParentWindow));
   if (parent && gotRestoredBounds) {
     int32_t parentX, parentY;
     if (NS_SUCCEEDED(parent->GetPosition(&parentX, &parentY))) {
       rect.MoveBy(-parentX, -parentY);
     }
   }
 
-  char                        sizeBuf[10];
   nsAutoString                sizeString;
   nsAutoString                windowElementId;
 
   // fetch docShellElement's ID and XUL owner document
   RefPtr<dom::XULDocument> ownerXULDoc =
     docShellElement->OwnerDoc()->AsXULDocument();
   if (docShellElement->IsXULElement()) {
     docShellElement->GetId(windowElementId);
   }
 
   bool shouldPersist = !isFullscreen && ownerXULDoc;
   ErrorResult rv;
   // (only for size elements which are persisted)
   if ((mPersistentAttributesDirty & PAD_POSITION) && gotRestoredBounds) {
     if (persistString.Find("screenX") >= 0) {
-      SprintfLiteral(sizeBuf, "%d", NSToIntRound(rect.X() / posScale.scale));
-      CopyASCIItoUTF16(sizeBuf, sizeString);
+      sizeString.Truncate();
+      sizeString.AppendInt(NSToIntRound(rect.X() / posScale.scale));
       docShellElement->SetAttribute(SCREENX_ATTRIBUTE, sizeString, rv);
       if (shouldPersist) {
         IgnoredErrorResult err;
         ownerXULDoc->Persist(windowElementId, SCREENX_ATTRIBUTE, err);
       }
     }
     if (persistString.Find("screenY") >= 0) {
-      SprintfLiteral(sizeBuf, "%d", NSToIntRound(rect.Y() / posScale.scale));
-      CopyASCIItoUTF16(sizeBuf, sizeString);
+      sizeString.Truncate();
+      sizeString.AppendInt(NSToIntRound(rect.Y() / posScale.scale));
       docShellElement->SetAttribute(SCREENY_ATTRIBUTE, sizeString, rv);
       if (shouldPersist) {
         IgnoredErrorResult err;
         ownerXULDoc->Persist(windowElementId, SCREENY_ATTRIBUTE, err);
       }
     }
   }
 
   if ((mPersistentAttributesDirty & PAD_SIZE) && gotRestoredBounds) {
     LayoutDeviceIntRect innerRect = rect - GetOuterToInnerSizeDifference(mWindow);
     if (persistString.Find("width") >= 0) {
-      SprintfLiteral(sizeBuf, "%d", NSToIntRound(innerRect.Width() / sizeScale.scale));
-      CopyASCIItoUTF16(sizeBuf, sizeString);
+      sizeString.Truncate();
+      sizeString.AppendInt(NSToIntRound(innerRect.Width() / sizeScale.scale));
       docShellElement->SetAttribute(WIDTH_ATTRIBUTE, sizeString, rv);
       if (shouldPersist) {
         IgnoredErrorResult err;
         ownerXULDoc->Persist(windowElementId, WIDTH_ATTRIBUTE, err);
       }
     }
     if (persistString.Find("height") >= 0) {
-      SprintfLiteral(sizeBuf, "%d", NSToIntRound(innerRect.Height() / sizeScale.scale));
-      CopyASCIItoUTF16(sizeBuf, sizeString);
+      sizeString.Truncate();
+      sizeString.AppendInt(NSToIntRound(innerRect.Height() / sizeScale.scale));
       docShellElement->SetAttribute(HEIGHT_ATTRIBUTE, sizeString, rv);
       if (shouldPersist) {
         IgnoredErrorResult err;
         ownerXULDoc->Persist(windowElementId, HEIGHT_ATTRIBUTE, err);
       }
     }
   }
 
@@ -1709,18 +1708,18 @@ NS_IMETHODIMP nsXULWindow::SavePersisten
         ownerXULDoc->Persist(windowElementId, MODE_ATTRIBUTE, err);
       }
     }
     if (persistString.Find("zlevel") >= 0) {
       uint32_t zLevel;
       nsCOMPtr<nsIWindowMediator> mediator(do_GetService(NS_WINDOWMEDIATOR_CONTRACTID));
       if (mediator) {
         mediator->GetZLevel(this, &zLevel);
-        SprintfLiteral(sizeBuf, "%" PRIu32, zLevel);
-        CopyASCIItoUTF16(sizeBuf, sizeString);
+        sizeString.Truncate();
+        sizeString.AppendInt(zLevel);
         docShellElement->SetAttribute(ZLEVEL_ATTRIBUTE, sizeString, rv);
         if (shouldPersist) {
           IgnoredErrorResult err;
           ownerXULDoc->Persist(windowElementId, ZLEVEL_ATTRIBUTE, err);
         }
       }
     }
   }