diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index 7547594b2c55..45164358a2b5 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -1369,7 +1369,7 @@ static bool NondeterministicGetWeakMapKeys(JSContext* cx, unsigned argc,
   return true;
 }
 
-class HasChildTracer : public JS::CallbackTracer {
+class HasChildTracer final : public JS::CallbackTracer {
   RootedValue child_;
   bool found_;
 
diff --git a/js/src/gc/DeletePolicy.h b/js/src/gc/DeletePolicy.h
index 4f66ef3d2f3e..5213af08f837 100644
--- a/js/src/gc/DeletePolicy.h
+++ b/js/src/gc/DeletePolicy.h
@@ -12,7 +12,7 @@
 namespace js {
 namespace gc {
 
-struct ClearEdgesTracer : public JS::CallbackTracer {
+struct ClearEdgesTracer final : public JS::CallbackTracer {
   ClearEdgesTracer();
 
 #ifdef DEBUG
diff --git a/js/src/gc/GC.cpp b/js/src/gc/GC.cpp
index 89d00b78c423..54a4452ca9af 100644
--- a/js/src/gc/GC.cpp
+++ b/js/src/gc/GC.cpp
@@ -4066,7 +4066,7 @@ bool GCRuntime::shouldPreserveJITCode(Realm* realm,
 }
 
 #ifdef DEBUG
-class CompartmentCheckTracer : public JS::CallbackTracer {
+class CompartmentCheckTracer final : public JS::CallbackTracer {
   void onChild(const JS::GCCellPtr& thing) override;
 
  public:
diff --git a/js/src/gc/GCInternals.h b/js/src/gc/GCInternals.h
index 484cc65dccd6..74548850a550 100644
--- a/js/src/gc/GCInternals.h
+++ b/js/src/gc/GCInternals.h
@@ -149,7 +149,7 @@ void CheckHashTablesAfterMovingGC(JSRuntime* rt);
 void CheckHeapAfterGC(JSRuntime* rt);
 #endif
 
-struct MovingTracer : JS::CallbackTracer {
+struct MovingTracer final : public JS::CallbackTracer {
   explicit MovingTracer(JSRuntime* rt)
       : CallbackTracer(rt, TraceWeakMapKeysValues) {}
 
diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index 1eaf4a41dbf5..d51a2a31c29d 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -3371,7 +3371,7 @@ FOR_EACH_PUBLIC_TAGGED_GC_POINTER_TYPE(INSTANTIATE_INTERNAL_MARKING_FUNCTIONS)
  */
 
 #ifdef DEBUG
-struct AssertNonGrayTracer : public JS::CallbackTracer {
+struct AssertNonGrayTracer final : public JS::CallbackTracer {
   explicit AssertNonGrayTracer(JSRuntime* rt) : JS::CallbackTracer(rt) {}
   void onChild(const JS::GCCellPtr& thing) override {
     MOZ_ASSERT(!thing.asCell()->isMarkedGray());
@@ -3379,7 +3379,7 @@ struct AssertNonGrayTracer : public JS::CallbackTracer {
 };
 #endif
 
-class UnmarkGrayTracer : public JS::CallbackTracer {
+class UnmarkGrayTracer final : public JS::CallbackTracer {
  public:
   // We set weakMapAction to DoNotTraceWeakMaps because the cycle collector
   // will fix up any color mismatches involving weakmaps when it runs.
diff --git a/js/src/gc/RootMarking.cpp b/js/src/gc/RootMarking.cpp
index 38761be2932f..c39a8f85eebb 100644
--- a/js/src/gc/RootMarking.cpp
+++ b/js/src/gc/RootMarking.cpp
@@ -415,7 +415,7 @@ void js::gc::GCRuntime::traceRuntimeCommon(JSTracer* trc,
 }
 
 #ifdef DEBUG
-class AssertNoRootsTracer : public JS::CallbackTracer {
+class AssertNoRootsTracer final : public JS::CallbackTracer {
   void onChild(const JS::GCCellPtr& thing) override {
     MOZ_CRASH("There should not be any roots after finishRoots");
   }
diff --git a/js/src/gc/Verifier.cpp b/js/src/gc/Verifier.cpp
index 4f7180925b21..f5e5154a27d2 100644
--- a/js/src/gc/Verifier.cpp
+++ b/js/src/gc/Verifier.cpp
@@ -266,7 +266,7 @@ static bool IsMarkedOrAllocated(TenuredCell* cell) {
   return cell->isMarkedAny();
 }
 
-struct CheckEdgeTracer : public JS::CallbackTracer {
+struct CheckEdgeTracer final : public JS::CallbackTracer {
   VerifyNode* node;
   explicit CheckEdgeTracer(JSRuntime* rt)
       : JS::CallbackTracer(rt), node(nullptr) {}
diff --git a/js/src/jsapi-tests/testGCMarking.cpp b/js/src/jsapi-tests/testGCMarking.cpp
index 317042f6e3b0..1f6b8310f48e 100644
--- a/js/src/jsapi-tests/testGCMarking.cpp
+++ b/js/src/jsapi-tests/testGCMarking.cpp
@@ -71,7 +71,7 @@ static bool ConstructCCW(JSContext* cx, const JSClass* globalClasp,
   return true;
 }
 
-class CCWTestTracer : public JS::CallbackTracer {
+class CCWTestTracer final : public JS::CallbackTracer {
   void onChild(const JS::GCCellPtr& thing) override {
     numberOfThingsTraced++;
 
diff --git a/js/src/jsapi-tests/testPrivateGCThingValue.cpp b/js/src/jsapi-tests/testPrivateGCThingValue.cpp
index 353306aee5d9..4fe18007dca4 100644
--- a/js/src/jsapi-tests/testPrivateGCThingValue.cpp
+++ b/js/src/jsapi-tests/testPrivateGCThingValue.cpp
@@ -15,7 +15,7 @@
 #include "js/SourceText.h"  // JS::Source{Ownership,Text}
 #include "jsapi-tests/tests.h"
 
-class TestTracer : public JS::CallbackTracer {
+class TestTracer final : public JS::CallbackTracer {
   void onChild(const JS::GCCellPtr& thing) override {
     if (thing.asCell() == expectedCell && thing.kind() == expectedKind) {
       found = true;
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index 6d3dc98a8ae4..16b81ab35e14 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -1006,7 +1006,7 @@ extern JS_FRIEND_API int JS::IsGCPoisoning() {
 #endif
 }
 
-struct DumpHeapTracer : public JS::CallbackTracer, public WeakMapTracer {
+struct DumpHeapTracer final : public JS::CallbackTracer, public WeakMapTracer {
   const char* prefix;
   FILE* output;
   mozilla::MallocSizeOf mallocSizeOf;
diff --git a/js/src/vm/UbiNode.cpp b/js/src/vm/UbiNode.cpp
index 118ee88d9eb6..f051eef0622c 100644
--- a/js/src/vm/UbiNode.cpp
+++ b/js/src/vm/UbiNode.cpp
@@ -191,7 +191,7 @@ Value Node::exposeToJS() const {
 
 // A JS::CallbackTracer subclass that adds a Edge to a Vector for each
 // edge on which it is invoked.
-class EdgeVectorTracer : public JS::CallbackTracer {
+class EdgeVectorTracer final : public JS::CallbackTracer {
   // The vector to which we add Edges.
   EdgeVector* vec;