--- a/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
+++ b/mozglue/baseprofiler/core/ProfileBufferEntry.cpp
@@ -201,17 +201,17 @@ UniqueJSONStrings::UniqueJSONStrings(con
   if (count != 0) {
     MOZ_RELEASE_ASSERT(mStringHashToIndexMap.reserve(count));
     for (auto iter = aOther.mStringHashToIndexMap.iter(); !iter.done();
          iter.next()) {
       mStringHashToIndexMap.putNewInfallible(iter.get().key(),
                                              iter.get().value());
     }
     UniquePtr<char[]> stringTableJSON =
-        aOther.mStringTableWriter.WriteFunc()->CopyData();
+        aOther.mStringTableWriter.ChunkedWriteFunc()->CopyData();
     mStringTableWriter.Splice(stringTableJSON.get());
   }
 }
 
 uint32_t UniqueJSONStrings::GetOrAddIndex(const char* aStr) {
   uint32_t count = mStringHashToIndexMap.count();
   HashNumber hash = HashString(aStr);
   auto entry = mStringHashToIndexMap.lookupForAdd(hash);
@@ -271,22 +271,22 @@ uint32_t UniqueStacks::GetOrAddFrameInde
 
   MOZ_RELEASE_ASSERT(mFrameToIndexMap.add(entry, aFrame, count));
   StreamNonJITFrame(aFrame);
   return count;
 }
 
 void UniqueStacks::SpliceFrameTableElements(SpliceableJSONWriter& aWriter) {
   mFrameTableWriter.EndBareList();
-  aWriter.TakeAndSplice(mFrameTableWriter.WriteFunc());
+  aWriter.TakeAndSplice(mFrameTableWriter.ChunkedWriteFunc());
 }
 
 void UniqueStacks::SpliceStackTableElements(SpliceableJSONWriter& aWriter) {
   mStackTableWriter.EndBareList();
-  aWriter.TakeAndSplice(mStackTableWriter.WriteFunc());
+  aWriter.TakeAndSplice(mStackTableWriter.ChunkedWriteFunc());
 }
 
 void UniqueStacks::StreamStack(const StackKey& aStack) {
   enum Schema : uint32_t { PREFIX = 0, FRAME = 1 };
 
   AutoArraySchemaWriter writer(mStackTableWriter, *mUniqueStrings);
   if (aStack.mPrefixStackIndex.isSome()) {
     writer.IntElement(PREFIX, *aStack.mPrefixStackIndex);

--- a/mozglue/baseprofiler/core/ProfileBufferEntry.h
+++ b/mozglue/baseprofiler/core/ProfileBufferEntry.h
@@ -138,17 +138,17 @@ class ProfileBufferEntry {
 static_assert(sizeof(ProfileBufferEntry) == 9, "bad ProfileBufferEntry size");
 
 class UniqueJSONStrings {
  public:
   UniqueJSONStrings();
   explicit UniqueJSONStrings(const UniqueJSONStrings& aOther);
 
   void SpliceStringTableElements(SpliceableJSONWriter& aWriter) {
-    aWriter.TakeAndSplice(mStringTableWriter.WriteFunc());
+    aWriter.TakeAndSplice(mStringTableWriter.ChunkedWriteFunc());
   }
 
   void WriteProperty(JSONWriter& aWriter, const char* aName, const char* aStr) {
     aWriter.IntProperty(aName, GetOrAddIndex(aStr));
   }
 
   void WriteElement(JSONWriter& aWriter, const char* aStr) {
     aWriter.IntElement(GetOrAddIndex(aStr));

--- a/mozglue/baseprofiler/core/platform.cpp
+++ b/mozglue/baseprofiler/core/platform.cpp
@@ -2791,30 +2791,30 @@ void profiler_set_process_name(const std
 UniquePtr<char[]> profiler_get_profile(double aSinceTime, bool aIsShuttingDown,
                                        bool aOnlyThreads) {
   LOG("profiler_get_profile");
 
   SpliceableChunkedJSONWriter b;
   if (!WriteProfileToJSONWriter(b, aSinceTime, aIsShuttingDown, aOnlyThreads)) {
     return nullptr;
   }
-  return b.WriteFunc()->CopyData();
+  return b.ChunkedWriteFunc()->CopyData();
 }
 
 void profiler_get_profile_json_into_lazily_allocated_buffer(
     const std::function<char*(size_t)>& aAllocator, double aSinceTime,
     bool aIsShuttingDown) {
   LOG("profiler_get_profile_json_into_lazily_allocated_buffer");
 
   SpliceableChunkedJSONWriter b;
   if (!WriteProfileToJSONWriter(b, aSinceTime, aIsShuttingDown)) {
     return;
   }
 
-  b.WriteFunc()->CopyDataIntoLazilyAllocatedBuffer(aAllocator);
+  b.ChunkedWriteFunc()->CopyDataIntoLazilyAllocatedBuffer(aAllocator);
 }
 
 void profiler_get_start_params(int* aCapacity, Maybe<double>* aDuration,
                                double* aInterval, uint32_t* aFeatures,
                                Vector<const char*>* aFilters) {
   MOZ_RELEASE_ASSERT(CorePS::Exists());
 
   if (!aCapacity || !aDuration || !aInterval || !aFeatures || !aFilters) {

--- a/mozglue/baseprofiler/public/BaseProfileJSONWriter.h
+++ b/mozglue/baseprofiler/public/BaseProfileJSONWriter.h
@@ -24,17 +24,17 @@ namespace baseprofiler {
 
 class SpliceableJSONWriter;
 
 // On average, profile JSONs are large enough such that we want to avoid
 // reallocating its buffer when expanding. Additionally, the contents of the
 // profile are not accessed until the profile is entirely written. For these
 // reasons we use a chunked writer that keeps an array of chunks, which is
 // concatenated together after writing is finished.
-class ChunkedJSONWriteFunc : public JSONWriteFunc {
+class ChunkedJSONWriteFunc final : public JSONWriteFunc {
  public:
   friend class SpliceableJSONWriter;
 
   ChunkedJSONWriteFunc() : mChunkPtr{nullptr}, mChunkEnd{nullptr} {
     AllocChunk(kChunkSize);
   }
 
   bool IsEmpty() const {
@@ -149,17 +149,17 @@ class ChunkedJSONWriteFunc : public JSON
   // List of chunks and their lengths.
   //
   // For all i, the length of the string in mChunkList[i] is
   // mChunkLengths[i].
   Vector<UniquePtr<char[]>> mChunkList;
   Vector<size_t> mChunkLengths;
 };
 
-struct OStreamJSONWriteFunc : public JSONWriteFunc {
+struct OStreamJSONWriteFunc final : public JSONWriteFunc {
   explicit OStreamJSONWriteFunc(std::ostream& aStream) : mStream(aStream) {}
 
   void Write(const char* aStr) override { mStream << aStr; }
   void Write(const char* aStr, size_t aLen) override { mStream << aStr; }
 
   std::ostream& mStream;
 };
 
@@ -203,29 +203,31 @@ class SpliceableJSONWriter : public JSON
     aFunc->mChunkPtr = nullptr;
     aFunc->mChunkEnd = nullptr;
     aFunc->mChunkList.clear();
     aFunc->mChunkLengths.clear();
     mNeedComma[mDepth] = true;
   }
 };
 
-class SpliceableChunkedJSONWriter : public SpliceableJSONWriter {
+class SpliceableChunkedJSONWriter final : public SpliceableJSONWriter {
  public:
   explicit SpliceableChunkedJSONWriter()
       : SpliceableJSONWriter(MakeUnique<ChunkedJSONWriteFunc>()) {}
 
-  ChunkedJSONWriteFunc* WriteFunc() const {
-    return static_cast<ChunkedJSONWriteFunc*>(JSONWriter::WriteFunc());
+  ChunkedJSONWriteFunc* ChunkedWriteFunc() const {
+    // The WriteFunc was initialized with a ChunkedJSONWriteFunc in the only
+    // constructor above, so it's safe to cast to ChunkedJSONWriteFunc*.
+    return static_cast<ChunkedJSONWriteFunc*>(WriteFunc());
   }
 
   // Adopts the chunks from aFunc without copying.
-  virtual void TakeAndSplice(ChunkedJSONWriteFunc* aFunc) override {
+  void TakeAndSplice(ChunkedJSONWriteFunc* aFunc) override {
     Separator();
-    WriteFunc()->Take(std::move(*aFunc));
+    ChunkedWriteFunc()->Take(std::move(*aFunc));
     mNeedComma[mDepth] = true;
   }
 };
 
 class JSONSchemaWriter {
   JSONWriter& mWriter;
   uint32_t mIndex;
 

--- a/tools/profiler/core/ProfileBufferEntry.cpp
+++ b/tools/profiler/core/ProfileBufferEntry.cpp
@@ -213,17 +213,17 @@ UniqueJSONStrings::UniqueJSONStrings(con
   if (count != 0) {
     MOZ_RELEASE_ASSERT(mStringHashToIndexMap.reserve(count));
     for (auto iter = aOther.mStringHashToIndexMap.iter(); !iter.done();
          iter.next()) {
       mStringHashToIndexMap.putNewInfallible(iter.get().key(),
                                              iter.get().value());
     }
     UniquePtr<char[]> stringTableJSON =
-        aOther.mStringTableWriter.WriteFunc()->CopyData();
+        aOther.mStringTableWriter.ChunkedWriteFunc()->CopyData();
     mStringTableWriter.Splice(stringTableJSON.get());
   }
 }
 
 uint32_t UniqueJSONStrings::GetOrAddIndex(const char* aStr) {
   uint32_t count = mStringHashToIndexMap.count();
   HashNumber hash = HashString(aStr);
   auto entry = mStringHashToIndexMap.lookupForAdd(hash);
@@ -375,22 +375,22 @@ uint32_t UniqueStacks::GetOrAddFrameInde
 
   MOZ_RELEASE_ASSERT(mFrameToIndexMap.add(entry, aFrame, count));
   StreamNonJITFrame(aFrame);
   return count;
 }
 
 void UniqueStacks::SpliceFrameTableElements(SpliceableJSONWriter& aWriter) {
   mFrameTableWriter.EndBareList();
-  aWriter.TakeAndSplice(mFrameTableWriter.WriteFunc());
+  aWriter.TakeAndSplice(mFrameTableWriter.ChunkedWriteFunc());
 }
 
 void UniqueStacks::SpliceStackTableElements(SpliceableJSONWriter& aWriter) {
   mStackTableWriter.EndBareList();
-  aWriter.TakeAndSplice(mStackTableWriter.WriteFunc());
+  aWriter.TakeAndSplice(mStackTableWriter.ChunkedWriteFunc());
 }
 
 void UniqueStacks::StreamStack(const StackKey& aStack) {
   enum Schema : uint32_t { PREFIX = 0, FRAME = 1 };
 
   AutoArraySchemaWriter writer(mStackTableWriter, *mUniqueStrings);
   if (aStack.mPrefixStackIndex.isSome()) {
     writer.IntElement(PREFIX, *aStack.mPrefixStackIndex);

--- a/tools/profiler/core/ProfileBufferEntry.h
+++ b/tools/profiler/core/ProfileBufferEntry.h
@@ -146,17 +146,17 @@ class ProfileBufferEntry {
 static_assert(sizeof(ProfileBufferEntry) == 9, "bad ProfileBufferEntry size");
 
 class UniqueJSONStrings {
  public:
   UniqueJSONStrings();
   explicit UniqueJSONStrings(const UniqueJSONStrings& aOther);
 
   void SpliceStringTableElements(SpliceableJSONWriter& aWriter) {
-    aWriter.TakeAndSplice(mStringTableWriter.WriteFunc());
+    aWriter.TakeAndSplice(mStringTableWriter.ChunkedWriteFunc());
   }
 
   void WriteProperty(mozilla::JSONWriter& aWriter, const char* aName,
                      const char* aStr) {
     aWriter.IntProperty(aName, GetOrAddIndex(aStr));
   }
 
   void WriteElement(mozilla::JSONWriter& aWriter, const char* aStr) {

--- a/tools/profiler/core/platform.cpp
+++ b/tools/profiler/core/platform.cpp
@@ -4224,34 +4224,34 @@ UniquePtr<char[]> profiler_get_profile(d
   UniquePtr<ProfilerCodeAddressService> service =
       profiler_code_address_service_for_presymbolication();
 
   SpliceableChunkedJSONWriter b;
   if (!WriteProfileToJSONWriter(b, aSinceTime, aIsShuttingDown,
                                 service.get())) {
     return nullptr;
   }
-  return b.WriteFunc()->CopyData();
+  return b.ChunkedWriteFunc()->CopyData();
 }
 
 void profiler_get_profile_json_into_lazily_allocated_buffer(
     const std::function<char*(size_t)>& aAllocator, double aSinceTime,
     bool aIsShuttingDown) {
   LOG("profiler_get_profile_json_into_lazily_allocated_buffer");
 
   UniquePtr<ProfilerCodeAddressService> service =
       profiler_code_address_service_for_presymbolication();
 
   SpliceableChunkedJSONWriter b;
   if (!WriteProfileToJSONWriter(b, aSinceTime, aIsShuttingDown,
                                 service.get())) {
     return;
   }
 
-  b.WriteFunc()->CopyDataIntoLazilyAllocatedBuffer(aAllocator);
+  b.ChunkedWriteFunc()->CopyDataIntoLazilyAllocatedBuffer(aAllocator);
 }
 
 void profiler_get_start_params(int* aCapacity, Maybe<double>* aDuration,
                                double* aInterval, uint32_t* aFeatures,
                                Vector<const char*>* aFilters,
                                uint64_t* aActiveBrowsingContextID) {
   MOZ_RELEASE_ASSERT(CorePS::Exists());
 

--- a/tools/profiler/gecko/nsProfiler.cpp
+++ b/tools/profiler/gecko/nsProfiler.cpp
@@ -992,17 +992,17 @@ void nsProfiler::FinishGathering() {
   MOZ_RELEASE_ASSERT(mPromiseHolder.isSome());
 
   // Close the "processes" array property.
   mWriter->EndArray();
 
   // Close the root object of the generated JSON.
   mWriter->End();
 
-  UniquePtr<char[]> buf = mWriter->WriteFunc()->CopyData();
+  UniquePtr<char[]> buf = mWriter->ChunkedWriteFunc()->CopyData();
   size_t len = strlen(buf.get());
   nsCString result;
   result.Adopt(buf.release(), len);
   mPromiseHolder->Resolve(std::move(result), __func__);
 
   ResetGathering();
 }
 

--- a/tools/profiler/tests/gtest/GeckoProfiler.cpp
+++ b/tools/profiler/tests/gtest/GeckoProfiler.cpp
@@ -835,17 +835,17 @@ TEST(GeckoProfiler, Markers)
   // The GTestMarkerPayloads should have been deserialized, streamed, and
   // destroyed.
   EXPECT_EQ(GTestMarkerPayload::sNumCreated, 10 + 0);
   EXPECT_EQ(GTestMarkerPayload::sNumSerialized, 10 + 0);
   EXPECT_EQ(GTestMarkerPayload::sNumDeserialized, 0 + 10);
   EXPECT_EQ(GTestMarkerPayload::sNumStreamed, 0 + 10);
   EXPECT_EQ(GTestMarkerPayload::sNumDestroyed, 10 + 10);
 
-  UniquePtr<char[]> profile = w.WriteFunc()->CopyData();
+  UniquePtr<char[]> profile = w.ChunkedWriteFunc()->CopyData();
   ASSERT_TRUE(!!profile.get());
 
   // Expected markers, in order.
   enum State {
     S_tracing_event,
     S_tracing_start,
     S_tracing_end,
     S_tracing_event_with_stack,
@@ -1538,30 +1538,30 @@ TEST(GeckoProfiler, Counters)
   PR_Sleep(PR_MillisecondsToInterval(200));
   AUTO_PROFILER_COUNT_TOTAL(TestCounter, -17);
   PR_Sleep(PR_MillisecondsToInterval(200));
 
   // Verify we got counters in the output
   SpliceableChunkedJSONWriter w;
   ASSERT_TRUE(::profiler_stream_json_for_this_process(w));
 
-  UniquePtr<char[]> profile = w.WriteFunc()->CopyData();
+  UniquePtr<char[]> profile = w.ChunkedWriteFunc()->CopyData();
 
   // counter name and description should appear as is.
   ASSERT_TRUE(strstr(profile.get(), COUNTER_NAME));
   ASSERT_TRUE(strstr(profile.get(), COUNTER_DESCRIPTION));
   ASSERT_FALSE(strstr(profile.get(), COUNTER_NAME2));
   ASSERT_FALSE(strstr(profile.get(), COUNTER_DESCRIPTION2));
 
   AUTO_PROFILER_COUNT_TOTAL(TestCounter2, 10);
   PR_Sleep(PR_MillisecondsToInterval(200));
 
   ASSERT_TRUE(::profiler_stream_json_for_this_process(w));
 
-  profile = w.WriteFunc()->CopyData();
+  profile = w.ChunkedWriteFunc()->CopyData();
   ASSERT_TRUE(strstr(profile.get(), COUNTER_NAME));
   ASSERT_TRUE(strstr(profile.get(), COUNTER_DESCRIPTION));
   ASSERT_TRUE(strstr(profile.get(), COUNTER_NAME2));
   ASSERT_TRUE(strstr(profile.get(), COUNTER_DESCRIPTION2));
 
   profiler_stop();
 }
 
@@ -1638,17 +1638,17 @@ TEST(GeckoProfiler, StreamJSONForThisPro
 
   profiler_start(PROFILER_DEFAULT_ENTRIES, PROFILER_DEFAULT_INTERVAL, features,
                  filters, MOZ_ARRAY_LENGTH(filters), 0);
 
   w.Start();
   ASSERT_TRUE(::profiler_stream_json_for_this_process(w));
   w.End();
 
-  UniquePtr<char[]> profile = w.WriteFunc()->CopyData();
+  UniquePtr<char[]> profile = w.ChunkedWriteFunc()->CopyData();
 
   JSONOutputCheck(profile.get());
 
   profiler_stop();
 
   ASSERT_TRUE(!::profiler_stream_json_for_this_process(w));
 }
 
@@ -1675,17 +1675,17 @@ TEST(GeckoProfiler, StreamJSONForThisPro
           "GeckoProfiler_StreamJSONForThisProcessThreaded_Test::TestBody",
           [&]() {
             w.Start();
             ASSERT_TRUE(::profiler_stream_json_for_this_process(w));
             w.End();
           }),
       NS_DISPATCH_SYNC);
 
-  UniquePtr<char[]> profile = w.WriteFunc()->CopyData();
+  UniquePtr<char[]> profile = w.ChunkedWriteFunc()->CopyData();
 
   JSONOutputCheck(profile.get());
 
   // Stop the profiler and call profiler_stream_json_for_this_process on a
   // background thread.
   thread->Dispatch(
       NS_NewRunnableFunction(
           "GeckoProfiler_StreamJSONForThisProcessThreaded_Test::TestBody",