--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -611,22 +611,24 @@ struct GCGraph
     {
         mNodes.Clear();
         mEdges.Clear();
         mWeakMaps.Clear();
         mRootCount = 0;
     }
 
     void SizeOfExcludingThis(MallocSizeOf aMallocSizeOf,
-                             size_t *aNodesSize, size_t *aEdgesSize) const {
+                             size_t *aNodesSize, size_t *aEdgesSize,
+                             size_t *aWeakMapsSize) const {
         *aNodesSize = mNodes.SizeOfExcludingThis(aMallocSizeOf);
         *aEdgesSize = mEdges.SizeOfExcludingThis(aMallocSizeOf);
 
-        // These fields are deliberately not measured:
-        // - mWeakMaps entries, because the pointers are non-owning
+        // We don't measure what the WeakMappings point to, because the
+        // pointers are non-owning.
+        *aWeakMapsSize = mWeakMaps.SizeOfExcludingThis(aMallocSizeOf);
     }
 };
 
 static nsISupports *
 CanonicalizeXPCOMParticipant(nsISupports *in)
 {
     nsISupports* out;
     in->QueryInterface(NS_GET_IID(nsCycleCollectionISupports),
@@ -971,16 +973,17 @@ public:
                  nsCycleCollectorResults *aResults,
                  nsICycleCollectorListener *aListener);
     void Shutdown();
 
     void SizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
                              size_t *aObjectSize,
                              size_t *aGraphNodesSize,
                              size_t *aGraphEdgesSize,
+                             size_t *aWeakMapsSize,
                              size_t *aWhiteNodeSize,
                              size_t *aPurpleBufferSize) const;
 
 private:
     void CheckThreadSafety();
     void ShutdownCollect(nsICycleCollectorListener *aListener);
 
     void PrepareForCollection(nsCycleCollectorResults *aResults,
@@ -2400,21 +2403,23 @@ class CycleCollectorMultiReporter MOZ_FI
     {
         name.AssignLiteral("cycle-collector");
         return NS_OK;
     }
 
     NS_IMETHOD CollectReports(nsIMemoryMultiReporterCallback* aCb,
                               nsISupports* aClosure)
     {
-        size_t objectSize, graphNodesSize, graphEdgesSize, whiteNodesSize,
-               purpleBufferSize;
+        size_t objectSize, graphNodesSize, graphEdgesSize, weakMapsSize,
+            whiteNodesSize, purpleBufferSize;
         mCollector->SizeOfIncludingThis(MallocSizeOf,
-                                        &objectSize, &graphNodesSize,
-                                        &graphEdgesSize, &whiteNodesSize,
+                                        &objectSize,
+                                        &graphNodesSize, &graphEdgesSize,
+                                        &weakMapsSize,
+                                        &whiteNodesSize,
                                         &purpleBufferSize);
 
     #define REPORT(_path, _amount, _desc)                                     \
         do {                                                                  \
             size_t amount = _amount;  /* evaluate |_amount| only once */      \
             if (amount > 0) {                                                 \
                 nsresult rv;                                                  \
                 rv = aCb->Callback(EmptyCString(), NS_LITERAL_CSTRING(_path), \
@@ -2431,16 +2436,21 @@ class CycleCollectorMultiReporter MOZ_FI
         REPORT("explicit/cycle-collector/graph-nodes", graphNodesSize,
                "Memory used for the nodes of the cycle collector's graph. "
                "This should be zero when the collector is idle.");
 
         REPORT("explicit/cycle-collector/graph-edges", graphEdgesSize,
                "Memory used for the edges of the cycle collector's graph. "
                "This should be zero when the collector is idle.");
 
+        REPORT("explicit/cycle-collector/weak-maps", weakMapsSize,
+               "Memory used for the representation of weak maps in the "
+               "cycle collector's graph. "
+               "This should be zero when the collector is idle.");
+
         REPORT("explicit/cycle-collector/white-nodes", whiteNodesSize,
                "Memory used for the cycle collector's white nodes array. "
                "This should be zero when the collector is idle.");
 
         REPORT("explicit/cycle-collector/purple-buffer", purpleBufferSize,
                "Memory used for the cycle collector's purple buffer.");
 
     #undef REPORT
@@ -2812,22 +2822,24 @@ nsCycleCollector::Shutdown()
     }
 }
 
 void
 nsCycleCollector::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf,
                                       size_t *aObjectSize,
                                       size_t *aGraphNodesSize,
                                       size_t *aGraphEdgesSize,
+                                      size_t *aWeakMapsSize,
                                       size_t *aWhiteNodeSize,
                                       size_t *aPurpleBufferSize) const
 {
     *aObjectSize = aMallocSizeOf(this);
 
-    mGraph.SizeOfExcludingThis(aMallocSizeOf, aGraphNodesSize, aGraphEdgesSize);
+    mGraph.SizeOfExcludingThis(aMallocSizeOf, aGraphNodesSize, aGraphEdgesSize,
+                               aWeakMapsSize);
 
     // No need to measure what the entries point to; the pointers are
     // non-owning.
     *aWhiteNodeSize = mWhiteNodes
                     ? mWhiteNodes->SizeOfIncludingThis(aMallocSizeOf)
                     : 0;
 
     *aPurpleBufferSize = mPurpleBuf.SizeOfExcludingThis(aMallocSizeOf);