--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -951,37 +951,16 @@ static nsISupports*
 CanonicalizeXPCOMParticipant(nsISupports* aIn)
 {
   nsISupports* out = nullptr;
   aIn->QueryInterface(NS_GET_IID(nsCycleCollectionISupports),
                       reinterpret_cast<void**>(&out));
   return out;
 }
 
-static inline void
-ToParticipant(nsISupports* aPtr, nsXPCOMCycleCollectionParticipant** aCp);
-
-static void
-CanonicalizeParticipant(void** aParti, nsCycleCollectionParticipant** aCp)
-{
-  // If the participant is null, this is an nsISupports participant,
-  // so we must QI to get the real participant.
-
-  if (!*aCp) {
-    nsISupports* nsparti = static_cast<nsISupports*>(*aParti);
-    nsparti = CanonicalizeXPCOMParticipant(nsparti);
-    NS_ASSERTION(nsparti,
-                 "Don't add objects that don't participate in collection!");
-    nsXPCOMCycleCollectionParticipant* xcp;
-    ToParticipant(nsparti, &xcp);
-    *aParti = nsparti;
-    *aCp = xcp;
-  }
-}
-
 struct nsPurpleBufferEntry
 {
   nsPurpleBufferEntry(void* aObject, nsCycleCollectingAutoRefCnt* aRefCnt,
                       nsCycleCollectionParticipant* aParticipant)
     : mObject(aObject)
     , mRefCnt(aRefCnt)
     , mParticipant(aParticipant)
   {
@@ -1421,16 +1400,31 @@ ToParticipant(nsISupports* aPtr, nsXPCOM
   // We use QI to move from an nsISupports to an
   // nsXPCOMCycleCollectionParticipant, which is a per-class singleton helper
   // object that implements traversal and unlinking logic for the nsISupports
   // in question.
   *aCp = nullptr;
   CallQueryInterface(aPtr, aCp);
 }
 
+static void
+ToParticipant(void* aParti, nsCycleCollectionParticipant** aCp)
+{
+  // If the participant is null, this is an nsISupports participant,
+  // so we must QI to get the real participant.
+
+  if (!*aCp) {
+    nsISupports* nsparti = static_cast<nsISupports*>(aParti);
+    MOZ_ASSERT(CanonicalizeXPCOMParticipant(nsparti) == nsparti);
+    nsXPCOMCycleCollectionParticipant* xcp;
+    ToParticipant(nsparti, &xcp);
+    *aCp = xcp;
+  }
+}
+
 template<class Visitor>
 MOZ_NEVER_INLINE void
 GraphWalker<Visitor>::Walk(PtrInfo* aPi)
 {
   nsDeque queue;
   CheckedPush(queue, aPi);
   DoWalk(queue);
 }
@@ -2229,17 +2223,17 @@ CCGraphBuilder::AddNode(void* aPtr, nsCy
                "nsCycleCollectionParticipant shouldn't change!");
   }
   return result;
 }
 
 bool
 CCGraphBuilder::AddPurpleRoot(void* aRoot, nsCycleCollectionParticipant* aParti)
 {
-  CanonicalizeParticipant(&aRoot, &aParti);
+  ToParticipant(aRoot, &aParti);
 
   if (WantAllTraces() || !aParti->CanSkipInCC(aRoot)) {
     PtrInfo* pinfo = AddNode(aRoot, aParti);
     if (!pinfo) {
       return false;
     }
   }
 
@@ -2665,17 +2659,17 @@ public:
 
   bool
   Visit(nsPurpleBuffer& aBuffer, nsPurpleBufferEntry* aEntry)
   {
     MOZ_ASSERT(aEntry->mObject, "Null object in purple buffer");
     if (!aEntry->mRefCnt->get()) {
       void* o = aEntry->mObject;
       nsCycleCollectionParticipant* cp = aEntry->mParticipant;
-      CanonicalizeParticipant(&o, &cp);
+      ToParticipant(o, &cp);
       SnowWhiteObject swo = { o, cp, aEntry->mRefCnt };
       mObjects.InfallibleAppend(swo);
       aBuffer.Remove(aEntry);
     }
     return true;
   }
 
   bool HasSnowWhiteObjects() const
@@ -2791,17 +2785,17 @@ public:
       } else if (!mDispatchedDeferredDeletion) {
         mDispatchedDeferredDeletion = true;
         nsCycleCollector_dispatchDeferredDeletion(false);
       }
       return true;
     }
     void* o = aEntry->mObject;
     nsCycleCollectionParticipant* cp = aEntry->mParticipant;
-    CanonicalizeParticipant(&o, &cp);
+    ToParticipant(o, &cp);
     if (aEntry->mRefCnt->IsPurple() && !cp->CanSkip(o, false) &&
         (!mRemoveChildlessNodes || MayHaveChild(o, cp))) {
       return true;
     }
     aBuffer.Remove(aEntry);
     return true;
   }
 
@@ -2998,20 +2992,19 @@ public:
   Visit(nsPurpleBuffer& aBuffer, nsPurpleBufferEntry* aEntry)
   {
     MOZ_ASSERT(aEntry->mObject,
                "Entries with null mObject shouldn't be in the purple buffer.");
     MOZ_ASSERT(aEntry->mRefCnt->get() != 0,
                "Snow-white objects shouldn't be in the purple buffer.");
 
     void* obj = aEntry->mObject;
-    if (!aEntry->mParticipant) {
-      obj = CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj));
-      MOZ_ASSERT(obj, "Don't add objects that don't participate in collection!");
-    }
+
+    MOZ_ASSERT(aEntry->mParticipant || CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(obj)) == obj,
+               "Suspect nsISupports pointer must be canonical");
 
     PtrInfo* pi = mGraph.FindNode(obj);
     if (!pi) {
       return true;
     }
     MOZ_ASSERT(pi->mParticipant, "No dead objects should be in the purple buffer.");
     if (MOZ_UNLIKELY(mLogger)) {
       mLogger->NoteIncrementalRoot((uint64_t)pi->mPointer);
@@ -3484,16 +3477,19 @@ nsCycleCollector::Suspect(void* aPtr, ns
     return;
   }
 
   MOZ_ASSERT(aPtr, "Don't suspect null pointers");
 
   MOZ_ASSERT(HasParticipant(aPtr, aParti),
              "Suspected nsISupports pointer must QI to nsXPCOMCycleCollectionParticipant");
 
+  MOZ_ASSERT(aParti || CanonicalizeXPCOMParticipant(static_cast<nsISupports*>(aPtr)) == aPtr,
+             "Suspect nsISupports pointer must be canonical");
+
   mPurpleBuf.Put(aPtr, aParti, aRefCnt);
 }
 
 void
 nsCycleCollector::CheckThreadSafety()
 {
 #ifdef DEBUG
   MOZ_ASSERT(mEventTarget->IsOnCurrentThread());
@@ -4004,17 +4000,17 @@ CycleCollectedJSContext::Get()
 MOZ_NEVER_INLINE static void
 SuspectAfterShutdown(void* aPtr, nsCycleCollectionParticipant* aCp,
                      nsCycleCollectingAutoRefCnt* aRefCnt,
                      bool* aShouldDelete)
 {
   if (aRefCnt->get() == 0) {
     if (!aShouldDelete) {
       // The CC is shut down, so we can't be in the middle of an ICC.
-      CanonicalizeParticipant(&aPtr, &aCp);
+      ToParticipant(aPtr, &aCp);
       aRefCnt->stabilizeForDeletion();
       aCp->DeleteCycleCollectable(aPtr);
     } else {
       *aShouldDelete = true;
     }
   } else {
     // Make sure we'll get called again.
     aRefCnt->RemoveFromPurpleBuffer();