diff --git a/xpcom/glue/nsCycleCollectionParticipant.h b/xpcom/glue/nsCycleCollectionParticipant.h
index 67842b493e..0e0e4b608a 100644
--- a/xpcom/glue/nsCycleCollectionParticipant.h
+++ b/xpcom/glue/nsCycleCollectionParticipant.h
@@ -11,12 +11,17 @@
 #include "nsCycleCollectionNoteChild.h"
 #include "js/RootingAPI.h"
 
+/**
+ * Note: the two IIDs below only differ in one bit in the last byte. This
+ * is an intentional hack in order to speed up the comparison inside
+ * NS_INTERFACE_TABLE_HEAD_CYCLE_COLLECTION_INHERITED.
+ */
 #define NS_XPCOMCYCLECOLLECTIONPARTICIPANT_IID                                 \
 {                                                                              \
-    0x9674489b,                                                                \
-    0x1f6f,                                                                    \
-    0x4550,                                                                    \
-    { 0xa7, 0x30, 0xcc, 0xae, 0xdd, 0x10, 0x4c, 0xf9 }                         \
+    0xc61eac14,                                                                \
+    0x5f7a,                                                                    \
+    0x4481,                                                                    \
+    { 0x96, 0x5e, 0x7e, 0xaa, 0x6e, 0xff, 0xa8, 0x5e }                         \
 }
 
 /**
@@ -263,8 +268,33 @@ NS_DEFINE_STATIC_IID_ACCESSOR(nsXPCOMCycleCollectionParticipant,
     return NS_OK;                                                              \
   } else
 
+// The IIDs for nsXPCOMCycleCollectionParticipant and nsCycleCollectionISupports
+// are special in that they only differ in their last byte. This allows for the
+// optimization below where we first check the first three words of the IID and
+// if we find a match we check the last word to decide which case we have to
+// deal with.
 #define NS_INTERFACE_MAP_ENTRY_CYCLE_COLLECTION(_class)                        \
-  NS_IMPL_QUERY_CYCLE_COLLECTION(_class)
+  if (TopThreeWordsEquals(aIID, NS_GET_IID(nsXPCOMCycleCollectionParticipant), \
+                          NS_GET_IID(nsCycleCollectionISupports)) &&           \
+       /* The calls to LowWordEquals here are repeated inside the if branch.   \
+          This is due to the fact that we need to maintain the if/else chain   \
+          for these macros, so that the control flow never enters the if branch\
+          unless if we're certain one of the LowWordEquals() branches will get \
+          executed. */                                                         \
+      (LowWordEquals(aIID, NS_GET_IID(nsXPCOMCycleCollectionParticipant)) ||   \
+       LowWordEquals(aIID, NS_GET_IID(nsCycleCollectionISupports)))) {         \
+    if (LowWordEquals(aIID, NS_GET_IID(nsXPCOMCycleCollectionParticipant))) {  \
+      *aInstancePtr = NS_CYCLE_COLLECTION_PARTICIPANT(_class);                 \
+      return NS_OK;                                                            \
+    }                                                                          \
+    if (LowWordEquals(aIID, NS_GET_IID(nsCycleCollectionISupports))) {         \
+      *aInstancePtr = NS_CYCLE_COLLECTION_CLASSNAME(_class)::Upcast(this);     \
+      return NS_OK;                                                            \
+    } else {                                                                   \
+      /* Avoid warnings about foundInterface being left uninitialized. */      \
+      foundInterface = nullptr;                                                \
+    }                                                                          \
+  } else
 
 #define NS_INTERFACE_MAP_ENTRY_CYCLE_COLLECTION_ISUPPORTS(_class)              \
   NS_IMPL_QUERY_CYCLE_COLLECTION_ISUPPORTS(_class)
@@ -279,21 +309,34 @@ NS_DEFINE_STATIC_IID_ACCESSOR(nsXPCOMCycleCollectionParticipant,
 
 #define NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION_INHERITED(_class)              \
   NS_INTERFACE_MAP_BEGIN(_class)                                               \
-    NS_INTERFACE_MAP_ENTRY_CYCLE_COLLECTION(_class)
+    NS_INTERFACE_MAP_ENTRY_CYCLE_COLLECTION(_class)                            \
+    NS_INTERFACE_MAP_ENTRY_CYCLE_COLLECTION_ISUPPORTS(_class)
 
 #define NS_INTERFACE_TABLE_TO_MAP_SEGUE_CYCLE_COLLECTION(_class)  \
   if (rv == NS_OK) return rv; \
   nsISupports* foundInterface; \
   NS_INTERFACE_MAP_ENTRIES_CYCLE_COLLECTION(_class)
 
+// The IIDs for nsXPCOMCycleCollectionParticipant and nsCycleCollectionISupports
+// are special in that they only differ in their last byte. This allows for the
+// optimization below where we first check the first three words of the IID and
+// if we find a match we check the last word to decide which case we have to
+// deal with.
 #define NS_INTERFACE_TABLE_HEAD_CYCLE_COLLECTION_INHERITED(_class)            \
   NS_IMETHODIMP _class::QueryInterface(REFNSIID aIID, void** aInstancePtr)    \
   {                                                                           \
     NS_PRECONDITION(aInstancePtr, "null out param");                          \
                                                                               \
-    if ( aIID.Equals(NS_GET_IID(nsXPCOMCycleCollectionParticipant)) ) {       \
-      *aInstancePtr = NS_CYCLE_COLLECTION_PARTICIPANT(_class);                \
-      return NS_OK;                                                           \
+    if (TopThreeWordsEquals(aIID, NS_GET_IID(nsXPCOMCycleCollectionParticipant), \
+                            NS_GET_IID(nsCycleCollectionISupports))) {        \
+      if (LowWordEquals(aIID, NS_GET_IID(nsXPCOMCycleCollectionParticipant))) { \
+        *aInstancePtr = NS_CYCLE_COLLECTION_PARTICIPANT(_class);              \
+        return NS_OK;                                                         \
+      }                                                                       \
+      if (LowWordEquals(aIID, NS_GET_IID(nsCycleCollectionISupports))) {      \
+        *aInstancePtr = NS_CYCLE_COLLECTION_CLASSNAME(_class)::Upcast(this);  \
+        return NS_OK;                                                         \
+      }                                                                       \
     }                                                                         \
     nsresult rv;
 
@@ -867,4 +910,32 @@ static NS_CYCLE_COLLECTION_INNERCLASS NS_CYCLE_COLLECTION_INNERNAME;
 
 #define NS_CYCLE_COLLECTION_NOTE_EDGE_NAME CycleCollectionNoteEdgeName
 
+// *** Fast IID comparison helper functions ***
+/**
+ * Equivalency of the high three words where two IIDs have the same
+ * top three words but not the same low word.
+ */
+inline bool TopThreeWordsEquals(const nsID& aID,
+                                const nsID& aOther1,
+                                const nsID& aOther2)
+{
+  MOZ_ASSERT((((uint32_t*)&aOther1.m0)[0] == ((uint32_t*)&aOther2.m0)[0]) &&
+             (((uint32_t*)&aOther1.m0)[1] == ((uint32_t*)&aOther2.m0)[1]) &&
+             (((uint32_t*)&aOther1.m0)[2] == ((uint32_t*)&aOther2.m0)[2]) &&
+             (((uint32_t*)&aOther1.m0)[3] != ((uint32_t*)&aOther2.m0)[3]));
+
+  return ((((uint32_t*)&aID.m0)[0] == ((uint32_t*)&aOther1.m0)[0]) &&
+          (((uint32_t*)&aID.m0)[1] == ((uint32_t*)&aOther1.m0)[1]) &&
+          (((uint32_t*)&aID.m0)[2] == ((uint32_t*)&aOther1.m0)[2]));
+}
+
+/**
+ * Equivalency of the fourth word where the two IIDs have the same
+ * top three words but not the same low word.
+ */
+inline bool LowWordEquals(const nsID& aID, const nsID& aOther)
+{
+  return (((uint32_t*)&aID.m0)[3] == ((uint32_t*)&aOther.m0)[3]);
+}
+
 #endif // nsCycleCollectionParticipant_h__