diff --git a/js/xpconnect/src/nsXPConnect.cpp b/js/xpconnect/src/nsXPConnect.cpp
index 6d113b34fc4..735457cfcee 100644
--- a/js/xpconnect/src/nsXPConnect.cpp
+++ b/js/xpconnect/src/nsXPConnect.cpp
@@ -388,6 +388,7 @@ struct NoteWeakMapChildrenTracer : public JSTracer
     {
     }
     nsCycleCollectionTraversalCallback &mCb;
+    bool mTracedAny;
     JSObject *mMap;
     void *mKey;
     void *mKeyDelegate;
@@ -406,6 +407,7 @@ TraceWeakMappingChild(JSTracer *trc, void **thingp, JSGCTraceKind kind)
         return;
     if (AddToCCKind(kind)) {
         tracer->mCb.NoteWeakMapping(tracer->mMap, tracer->mKey, tracer->mKeyDelegate, thing);
+        tracer->mTracedAny = true;
     } else {
         JS_TraceChildren(trc, thing, kind);
     }
@@ -430,10 +432,12 @@ TraceWeakMapping(js::WeakMapTracer *trc, JSObject *m,
 {
     MOZ_ASSERT(trc->callback == TraceWeakMapping);
     NoteWeakMapsTracer *tracer = static_cast<NoteWeakMapsTracer *>(trc);
-    if (vkind == JSTRACE_STRING)
-        return;
-    if (!xpc_IsGrayGCThing(v) && !tracer->mCb.WantAllTraces())
-        return;
+
+    // If nothing that could be held alive by this entry is marked gray, return.
+    if ((!k || !xpc_IsGrayGCThing(k)) && MOZ_LIKELY(!tracer->mCb.WantAllTraces())) {
+        if (!v || !xpc_IsGrayGCThing(v) || vkind == JSTRACE_STRING)
+            return;
+    }
 
     // The cycle collector can only properly reason about weak maps if it can
     // reason about the liveness of their keys, which in turn requires that
@@ -448,17 +452,25 @@ TraceWeakMapping(js::WeakMapTracer *trc, JSObject *m,
     if (!AddToCCKind(kkind))
         k = nullptr;
 
-    JSObject *kdelegate = NULL;
-    if (kkind == JSTRACE_OBJECT)
+    JSObject *kdelegate = nullptr;
+    if (k && kkind == JSTRACE_OBJECT)
         kdelegate = js::GetWeakmapKeyDelegate((JSObject *)k);
 
     if (AddToCCKind(vkind)) {
         tracer->mCb.NoteWeakMapping(m, k, kdelegate, v);
     } else {
+        tracer->mChildTracer.mTracedAny = false;
         tracer->mChildTracer.mMap = m;
         tracer->mChildTracer.mKey = k;
         tracer->mChildTracer.mKeyDelegate = kdelegate;
-        JS_TraceChildren(&tracer->mChildTracer, v, vkind);
+
+        if (v && vkind != JSTRACE_STRING)
+            JS_TraceChildren(&tracer->mChildTracer, v, vkind);
+
+        // The delegate could hold alive the key, so report something to the CC
+        // if we haven't already.
+        if (!tracer->mChildTracer.mTracedAny && k && xpc_IsGrayGCThing(k) && kdelegate)
+            tracer->mCb.NoteWeakMapping(m, k, kdelegate, nullptr);
     }
 }
 
diff --git a/xpcom/base/nsCycleCollector.cpp b/xpcom/base/nsCycleCollector.cpp
index 38696f22e25..3b9510edf87 100644
--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -2017,16 +2017,13 @@ GCGraphBuilder::AddWeakMapNode(void *node)
 NS_IMETHODIMP_(void)
 GCGraphBuilder::NoteWeakMapping(void *map, void *key, void *kdelegate, void *val)
 {
-    PtrInfo *valNode = AddWeakMapNode(val);
-
-    if (!valNode)
-        return;
-
+    // Don't try to optimize away the entry here, as we've already attempted to
+    // do that in TraceWeakMapping in nsXPConnect.
     WeakMapping *mapping = mWeakMaps.AppendElement();
     mapping->mMap = map ? AddWeakMapNode(map) : nullptr;
     mapping->mKey = key ? AddWeakMapNode(key) : nullptr;
     mapping->mKeyDelegate = kdelegate ? AddWeakMapNode(kdelegate) : mapping->mKey;
-    mapping->mVal = valNode;
+    mapping->mVal = val ? AddWeakMapNode(val) : nullptr;
 }
 
 static bool
@@ -2251,11 +2248,11 @@ nsCycleCollector::ScanWeakMaps()
         for (uint32_t i = 0; i < mGraph.mWeakMaps.Length(); i++) {
             WeakMapping *wm = &mGraph.mWeakMaps[i];
 
-            // If mMap or mKey are null, the original object was marked black.
+            // If any of these are null, the original object was marked black.
             uint32_t mColor = wm->mMap ? wm->mMap->mColor : black;
             uint32_t kColor = wm->mKey ? wm->mKey->mColor : black;
             uint32_t kdColor = wm->mKeyDelegate ? wm->mKeyDelegate->mColor : black;
-            PtrInfo *v = wm->mVal;
+            uint32_t vColor = wm->mVal ? wm->mVal->mColor : black;
 
             // All non-null weak mapping maps, keys and values are
             // roots (in the sense of WalkFromRoots) in the cycle
@@ -2264,15 +2261,15 @@ nsCycleCollector::ScanWeakMaps()
             MOZ_ASSERT(mColor != grey, "Uncolored weak map");
             MOZ_ASSERT(kColor != grey, "Uncolored weak map key");
             MOZ_ASSERT(kdColor != grey, "Uncolored weak map key delegate");
-            MOZ_ASSERT(v->mColor != grey, "Uncolored weak map value");
+            MOZ_ASSERT(vColor != grey, "Uncolored weak map value");
 
             if (mColor == black && kColor != black && kdColor == black) {
                 GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mWhiteNodeCount)).Walk(wm->mKey);
                 anyChanged = true;
             }
 
-            if (mColor == black && kColor == black && v->mColor != black) {
-                GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mWhiteNodeCount)).Walk(v);
+            if (mColor == black && kColor == black && vColor != black) {
+                GraphWalker<ScanBlackVisitor>(ScanBlackVisitor(mWhiteNodeCount)).Walk(wm->mVal);
                 anyChanged = true;
             }
         }