diff --git a/browser/app/winlauncher/ErrorHandler.cpp b/browser/app/winlauncher/ErrorHandler.cpp
index 43a1692d5442..1286e0f90f69 100644
--- a/browser/app/winlauncher/ErrorHandler.cpp
+++ b/browser/app/winlauncher/ErrorHandler.cpp
@@ -152,7 +152,7 @@ class TempFileWriter final : public mozilla::JSONWriteFunc {
 
   explicit operator bool() const { return !mFailed; }
 
-  void Write(const mozilla::Span<const char>& aStr) override {
+  void Write(const mozilla::Span<const char>& aStr) final {
     if (mFailed) {
       return;
     }
diff --git a/memory/replace/dmd/test/SmokeDMD.cpp b/memory/replace/dmd/test/SmokeDMD.cpp
index e36ae57d5048..cf54d4d4f388 100644
--- a/memory/replace/dmd/test/SmokeDMD.cpp
+++ b/memory/replace/dmd/test/SmokeDMD.cpp
@@ -32,7 +32,7 @@ using namespace mozilla::dmd;
 
 DMDFuncs::Singleton DMDFuncs::sSingleton;
 
-class FpWriteFunc : public mozilla::JSONWriteFunc {
+class FpWriteFunc final : public mozilla::JSONWriteFunc {
  public:
   explicit FpWriteFunc(const char* aFilename) {
     mFp = fopen(aFilename, "w");
@@ -45,7 +45,7 @@ class FpWriteFunc : public mozilla::JSONWriteFunc {
 
   ~FpWriteFunc() { fclose(mFp); }
 
-  void Write(const mozilla::Span<const char>& aStr) override {
+  void Write(const mozilla::Span<const char>& aStr) final {
     for (const char c : aStr) {
       fputc(c, mFp);
     }
diff --git a/mfbt/tests/TestJSONWriter.cpp b/mfbt/tests/TestJSONWriter.cpp
index 52454a5e23a5..a90732396f28 100644
--- a/mfbt/tests/TestJSONWriter.cpp
+++ b/mfbt/tests/TestJSONWriter.cpp
@@ -18,10 +18,10 @@ using mozilla::MakeUnique;
 using mozilla::Span;
 
 // This writes all the output into a big buffer.
-struct StringWriteFunc : public JSONWriteFunc {
+struct StringWriteFunc final : public JSONWriteFunc {
   std::string mString;
 
-  void Write(const mozilla::Span<const char>& aStr) override {
+  void Write(const mozilla::Span<const char>& aStr) final {
     mString.append(aStr.data(), aStr.size());
   }
 };
diff --git a/mozglue/baseprofiler/public/BaseProfileJSONWriter.h b/mozglue/baseprofiler/public/BaseProfileJSONWriter.h
index c6b996a4f658..50b21db75cba 100644
--- a/mozglue/baseprofiler/public/BaseProfileJSONWriter.h
+++ b/mozglue/baseprofiler/public/BaseProfileJSONWriter.h
@@ -52,7 +52,7 @@ class ChunkedJSONWriteFunc final : public JSONWriteFunc {
     return totalLen;
   }
 
-  void Write(const Span<const char>& aStr) override {
+  void Write(const Span<const char>& aStr) final {
     MOZ_ASSERT(mChunkPtr >= mChunkList.back().get() && mChunkPtr <= mChunkEnd);
     MOZ_ASSERT(mChunkEnd >= mChunkList.back().get() + mChunkLengths.back());
     MOZ_ASSERT(*mChunkPtr == '\0');
@@ -151,7 +151,7 @@ class ChunkedJSONWriteFunc final : public JSONWriteFunc {
 struct OStreamJSONWriteFunc final : public JSONWriteFunc {
   explicit OStreamJSONWriteFunc(std::ostream& aStream) : mStream(aStream) {}
 
-  void Write(const Span<const char>& aStr) override {
+  void Write(const Span<const char>& aStr) final {
     std::string_view sv(aStr.data(), aStr.size());
     mStream << sv;
   }
diff --git a/mozglue/tests/TestBaseProfiler.cpp b/mozglue/tests/TestBaseProfiler.cpp
index b347e123e29a..2fa2e161c8e2 100644
--- a/mozglue/tests/TestBaseProfiler.cpp
+++ b/mozglue/tests/TestBaseProfiler.cpp
@@ -1146,10 +1146,10 @@ void TestLEB128() {
   printf("TestLEB128 done\n");
 }
 
-struct StringWriteFunc : public JSONWriteFunc {
+struct StringWriteFunc final : public JSONWriteFunc {
   std::string mString;
 
-  void Write(const mozilla::Span<const char>& aStr) override {
+  void Write(const mozilla::Span<const char>& aStr) final {
     mString.append(aStr.data(), aStr.size());
   }
 };
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index bae8d2c4d2a1..adec2675f1cc 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -3094,11 +3094,11 @@ static nsresult SelectProfile(nsToolkitProfileService* aProfileSvc,
 }
 
 #ifdef MOZ_BLOCK_PROFILE_DOWNGRADE
-struct FileWriteFunc : public JSONWriteFunc {
+struct FileWriteFunc final : public JSONWriteFunc {
   FILE* mFile;
   explicit FileWriteFunc(FILE* aFile) : mFile(aFile) {}
 
-  void Write(const Span<const char>& aStr) override {
+  void Write(const Span<const char>& aStr) final {
     fprintf(mFile, "%.*s", int(aStr.size()), aStr.data());
   }
 };
diff --git a/xpcom/base/JSONStringWriteFuncs.h b/xpcom/base/JSONStringWriteFuncs.h
index 5db4fab30969..65b688788cbf 100644
--- a/xpcom/base/JSONStringWriteFuncs.h
+++ b/xpcom/base/JSONStringWriteFuncs.h
@@ -24,7 +24,7 @@ class JSONStringWriteFunc final : public JSONWriteFunc {
  public:
   JSONStringWriteFunc() = default;
 
-  void Write(const Span<const char>& aStr) override { mString.Append(aStr); }
+  void Write(const Span<const char>& aStr) final { mString.Append(aStr); }
 
   const StringType& StringCRef() const { return mString; }
 
@@ -39,7 +39,7 @@ class JSONStringRefWriteFunc final : public JSONWriteFunc {
  public:
   MOZ_IMPLICIT JSONStringRefWriteFunc(nsACString& aString) : mString(aString) {}
 
-  void Write(const Span<const char>& aStr) override { mString.Append(aStr); }
+  void Write(const Span<const char>& aStr) final { mString.Append(aStr); }
 
   const nsACString& StringCRef() const { return mString; }
 
diff --git a/xpcom/base/nsMemoryInfoDumper.cpp b/xpcom/base/nsMemoryInfoDumper.cpp
index 1c15e301a99e..28af408178e4 100644
--- a/xpcom/base/nsMemoryInfoDumper.cpp
+++ b/xpcom/base/nsMemoryInfoDumper.cpp
@@ -385,11 +385,11 @@ static void MakeFilename(const char* aPrefix, const nsAString& aIdentifier,
 // the following two problems:
 // - It provides a JSONWriterFunc::Write() that calls nsGZFileWriter::Write().
 // - It can be stored as a UniquePtr, whereas nsGZFileWriter is refcounted.
-class GZWriterWrapper : public JSONWriteFunc {
+class GZWriterWrapper final : public JSONWriteFunc {
  public:
   explicit GZWriterWrapper(nsGZFileWriter* aGZWriter) : mGZWriter(aGZWriter) {}
 
-  void Write(const Span<const char>& aStr) override {
+  void Write(const Span<const char>& aStr) final {
     // Ignore any failure because JSONWriteFunc doesn't have a mechanism for
     // handling errors.
     Unused << mGZWriter->Write(aStr.data(), aStr.size());