diff --git a/js/src/gc/Cell.h b/js/src/gc/Cell.h
index 6fa72f5568..b9243e0898 100644
--- a/js/src/gc/Cell.h
+++ b/js/src/gc/Cell.h
@@ -420,7 +420,8 @@ static MOZ_ALWAYS_INLINE void
 AssertValidToSkipBarrier(TenuredCell* thing)
 {
     MOZ_ASSERT(!IsInsideNursery(thing));
-    MOZ_ASSERT_IF(thing, MapAllocToTraceKind(thing->getAllocKind()) != JS::TraceKind::Object);
+    MOZ_ASSERT_IF(thing, MapAllocToTraceKind(thing->getAllocKind()) != JS::TraceKind::Object &&
+                         MapAllocToTraceKind(thing->getAllocKind()) != JS::TraceKind::String);
 }
 
 /* static */ MOZ_ALWAYS_INLINE void
diff --git a/js/src/gc/RelocationOverlay.h b/js/src/gc/RelocationOverlay.h
index 43e80f517f..a1f514d7c5 100644
--- a/js/src/gc/RelocationOverlay.h
+++ b/js/src/gc/RelocationOverlay.h
@@ -15,6 +15,9 @@
 
 #include <stdint.h>
 
+#include "js/HeapAPI.h"
+#include "vm/Shape.h"
+#include "jsobj.h"
 namespace js {
 namespace gc {
 
@@ -26,11 +29,22 @@ struct Cell;
  */
 class RelocationOverlay
 {
-    /* The low bit is set so this should never equal a normal pointer. */
-    static const uintptr_t Relocated = uintptr_t(0xbad0bad1);
+    /* See comment in js/public/HeapAPI.h. */
+    static const uint32_t Relocated = js::gc::Relocated;
+
+#if MOZ_LITTLE_ENDIAN
+    /*
+     * Keep the low 32 bits untouched. Use them to distinguish strings from
+     * objects in the nursery.
+     */
+    uint32_t preserve_;
 
     /* Set to Relocated when moved. */
-    uintptr_t magic_;
+    uint32_t magic_;
+#else
+    uint32_t magic_;
+    uint32_t preserve_;
+#endif
 
     /* The location |this| was moved to. */
     Cell* newLocation_;
@@ -39,16 +53,15 @@ class RelocationOverlay
     RelocationOverlay* next_;
 
   public:
+  static const RelocationOverlay* fromCell(const Cell* cell) {
+        return reinterpret_cast<const RelocationOverlay*>(cell);
+    }
     static RelocationOverlay* fromCell(Cell* cell) {
         return reinterpret_cast<RelocationOverlay*>(cell);
     }
 
-    // ADD THIS: Const-friendly version
-    static const RelocationOverlay* fromCell(const Cell* cell) {
-        return reinterpret_cast<const RelocationOverlay*>(cell);
-    }
-
     bool isForwarded() const {
+        (void) preserve_; // Suppress warning
         return magic_ == Relocated;
     }
 
@@ -69,7 +82,7 @@ class RelocationOverlay
         return next_;
     }
 
-    static bool isCellForwarded(Cell* cell) {
+    static bool isCellForwarded(const Cell* cell) {
         return fromCell(cell)->isForwarded();
     }
 };
diff --git a/js/src/gc/StoreBuffer-inl.h b/js/src/gc/StoreBuffer-inl.h
index 7ef6b10c5c..b244b7672f 100644
--- a/js/src/gc/StoreBuffer-inl.h
+++ b/js/src/gc/StoreBuffer-inl.h
@@ -10,6 +10,7 @@
 
 #include "gc/Cell.h"
 #include "gc/Heap.h"
+#include "gc/Cell.h"
 
 namespace js {
 namespace gc {