diff --git a/dom/reporting/ReportingHeader.cpp b/dom/reporting/ReportingHeader.cpp
index d22cc3d4eca2..66a7c93e1e63 100644
--- a/dom/reporting/ReportingHeader.cpp
+++ b/dom/reporting/ReportingHeader.cpp
@@ -32,8 +32,7 @@
 #define REPORTING_PURGE_ALL "reporting:purge-all"
 #define REPORTING_PURGE_HOST "reporting:purge-host"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 namespace {
 
@@ -777,5 +776,4 @@ NS_INTERFACE_MAP_END
 NS_IMPL_ADDREF(ReportingHeader)
 NS_IMPL_RELEASE(ReportingHeader)
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/reporting/ReportingObserver.cpp b/dom/reporting/ReportingObserver.cpp
index 43a8acc01037..a9ab0bfd46bd 100644
--- a/dom/reporting/ReportingObserver.cpp
+++ b/dom/reporting/ReportingObserver.cpp
@@ -11,8 +11,7 @@
 #include "nsIGlobalObject.h"
 #include "nsThreadUtils.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(ReportingObserver)
 NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(ReportingObserver)
@@ -154,5 +153,4 @@ void ReportingObserver::MaybeNotify() {
 
 void ReportingObserver::ForgetReports() { mReports.Clear(); }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/reporting/ReportingUtils.cpp b/dom/reporting/ReportingUtils.cpp
index 497606ccd93f..ce7966a0a343 100644
--- a/dom/reporting/ReportingUtils.cpp
+++ b/dom/reporting/ReportingUtils.cpp
@@ -14,8 +14,7 @@
 #include "nsAtom.h"
 #include "nsIGlobalObject.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 /* static */
 void ReportingUtils::Report(nsIGlobalObject* aGlobal, nsAtom* aType,
@@ -43,5 +42,4 @@ void ReportingUtils::Report(nsIGlobalObject* aGlobal, nsAtom* aType,
   ReportDeliver::Record(window, type, aGroupName, aURL, aBody);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/reporting/ReportingUtils.h b/dom/reporting/ReportingUtils.h
index 8ea06715e6fa..ee9726bed733 100644
--- a/dom/reporting/ReportingUtils.h
+++ b/dom/reporting/ReportingUtils.h
@@ -12,8 +12,7 @@
 class nsAtom;
 class nsIGlobalObject;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ReportBody;
 
@@ -24,7 +23,6 @@ class ReportingUtils final {
                      ReportBody* aBody);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ReportingUtils_h
diff --git a/dom/reporting/TestingDeprecatedInterface.cpp b/dom/reporting/TestingDeprecatedInterface.cpp
index 9a8d65be196c..9c03f3125dae 100644
--- a/dom/reporting/TestingDeprecatedInterface.cpp
+++ b/dom/reporting/TestingDeprecatedInterface.cpp
@@ -8,8 +8,7 @@
 #include "mozilla/dom/BindingDeclarations.h"
 #include "mozilla/dom/ReportingBinding.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(TestingDeprecatedInterface, mGlobal)
 
@@ -46,5 +45,4 @@ void TestingDeprecatedInterface::DeprecatedMethod() const {}
 
 bool TestingDeprecatedInterface::DeprecatedAttribute() const { return true; }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/reporting/TestingDeprecatedInterface.h b/dom/reporting/TestingDeprecatedInterface.h
index 224063fbf512..016da79fa478 100644
--- a/dom/reporting/TestingDeprecatedInterface.h
+++ b/dom/reporting/TestingDeprecatedInterface.h
@@ -17,8 +17,7 @@
 
 class nsIGlobalObject;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 class GlobalObject;
 
 class TestingDeprecatedInterface final : public nsISupports,
@@ -46,7 +45,6 @@ class TestingDeprecatedInterface final : public nsISupports,
   nsCOMPtr<nsIGlobalObject> mGlobal;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_TestingDeprecatedInterface_h
diff --git a/dom/script/ModuleLoader.h b/dom/script/ModuleLoader.h
index d4a3a25c824d..ff37a3199921 100644
--- a/dom/script/ModuleLoader.h
+++ b/dom/script/ModuleLoader.h
@@ -25,8 +25,7 @@ class ModuleLoadRequest;
 }  // namespace loader
 }  // namespace JS
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ScriptLoader;
 class SRIMetadata;
@@ -85,7 +84,6 @@ class ModuleLoader final : public JS::loader::ModuleLoaderBase {
   const Kind mKind;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ModuleLoader_h
diff --git a/dom/script/ScriptDecoding.h b/dom/script/ScriptDecoding.h
index f39818537428..ea4c4aeef3a9 100644
--- a/dom/script/ScriptDecoding.h
+++ b/dom/script/ScriptDecoding.h
@@ -19,8 +19,7 @@
 #include <stdint.h>     // uint8_t, uint32_t
 #include <type_traits>  // std::is_same
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 template <typename Unit>
 struct ScriptDecoding {
@@ -84,7 +83,6 @@ struct ScriptDecoding<Utf8Unit> {
   }
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ScriptDecoding_h
diff --git a/dom/script/ScriptElement.h b/dom/script/ScriptElement.h
index 6f0e69f7bd3f..241ae67eb425 100644
--- a/dom/script/ScriptElement.h
+++ b/dom/script/ScriptElement.h
@@ -12,8 +12,7 @@
 #include "nsIScriptElement.h"
 #include "nsStubMutationObserver.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 /**
  * Baseclass useful for script elements (such as <xhtml:script> and
@@ -48,7 +47,6 @@ class ScriptElement : public nsIScriptElement, public nsStubMutationObserver {
   virtual bool MaybeProcessScript() override;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ScriptElement_h
diff --git a/dom/script/ScriptLoadContext.cpp b/dom/script/ScriptLoadContext.cpp
index aa759caad78f..c4ceaf6060f4 100644
--- a/dom/script/ScriptLoadContext.cpp
+++ b/dom/script/ScriptLoadContext.cpp
@@ -24,8 +24,7 @@
 #include "nsIClassOfService.h"
 #include "nsISupportsPriority.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 //////////////////////////////////////////////////////////////
 // ScriptLoadContext
@@ -227,5 +226,4 @@ void ScriptLoadContext::GetProfilerLabel(nsACString& aOutString) {
   }
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/script/ScriptLoadContext.h b/dom/script/ScriptLoadContext.h
index 683d82f475c4..108295d9671d 100644
--- a/dom/script/ScriptLoadContext.h
+++ b/dom/script/ScriptLoadContext.h
@@ -35,8 +35,7 @@ namespace JS {
 class OffThreadToken;
 }  // namespace JS
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class Element;
 
@@ -192,7 +191,6 @@ class ScriptLoadContext : public JS::loader::LoadContextBase,
   nsresult mUnreportedPreloadError;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ScriptLoadContext_h
diff --git a/dom/script/ScriptLoadHandler.cpp b/dom/script/ScriptLoadHandler.cpp
index 452f619a1d1f..d27298f96275 100644
--- a/dom/script/ScriptLoadHandler.cpp
+++ b/dom/script/ScriptLoadHandler.cpp
@@ -39,8 +39,7 @@
 #include "nsString.h"
 #include "nsTArray.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 #undef LOG
 #define LOG(args) \
@@ -428,5 +427,4 @@ ScriptLoadHandler::OnStreamComplete(nsIIncrementalStreamLoader* aLoader,
 #undef LOG_ENABLED
 #undef LOG
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/security/CSPEvalChecker.h b/dom/security/CSPEvalChecker.h
index 912b70c62bcc..c2a1f8d774a8 100644
--- a/dom/security/CSPEvalChecker.h
+++ b/dom/security/CSPEvalChecker.h
@@ -12,8 +12,7 @@
 struct JSContext;
 class nsGlobalWindowInner;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class WorkerPrivate;
 
@@ -28,7 +27,6 @@ class CSPEvalChecker final {
                                  bool* aAllowEval);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_CSPEvalChecker_h
diff --git a/dom/security/FramingChecker.h b/dom/security/FramingChecker.h
index 5935c22da97d..17f0bef82f04 100644
--- a/dom/security/FramingChecker.h
+++ b/dom/security/FramingChecker.h
@@ -16,11 +16,9 @@ class nsIDocShellTreeItem;
 class nsIURI;
 class nsIContentSecurityPolicy;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 class BrowsingContext;
-}
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 class FramingChecker {
  public:
diff --git a/dom/security/ReferrerInfo.cpp b/dom/security/ReferrerInfo.cpp
index 103f7cddd428..3b25121a66d1 100644
--- a/dom/security/ReferrerInfo.cpp
+++ b/dom/security/ReferrerInfo.cpp
@@ -37,8 +37,7 @@ static mozilla::LazyLogModule gReferrerInfoLog("ReferrerInfo");
 
 using namespace mozilla::net;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 // Implementation of ClassInfo is required to serialize/deserialize
 NS_IMPL_CLASSINFO(ReferrerInfo, nullptr, nsIClassInfo::MAIN_THREAD_ONLY,
@@ -1587,5 +1586,4 @@ void ReferrerInfo::RecordTelemetry(nsIHttpChannel* aChannel) {
                         static_cast<uint32_t>(mPolicy) + telemetryOffset);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/security/ReferrerInfo.h b/dom/security/ReferrerInfo.h
index 9bdc4029773e..c9a0415bbbed 100644
--- a/dom/security/ReferrerInfo.h
+++ b/dom/security/ReferrerInfo.h
@@ -42,8 +42,7 @@ class nsHttpChannel;
 
 using mozilla::Maybe;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 /**
  * The ReferrerInfo class holds the raw referrer and potentially a referrer
@@ -471,7 +470,6 @@ class ReferrerInfo : public nsIReferrerInfo {
 #endif  // DEBUG
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ReferrerInfo_h
diff --git a/dom/security/SRICheck.cpp b/dom/security/SRICheck.cpp
index d397c60af164..79e8b9429afb 100644
--- a/dom/security/SRICheck.cpp
+++ b/dom/security/SRICheck.cpp
@@ -28,8 +28,7 @@
 #define SRIERROR(args) \
   MOZ_LOG(SRILogHelper::GetSriLog(), mozilla::LogLevel::Error, args)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 /**
  * Returns whether or not the sub-resource about to be loaded is eligible
@@ -491,5 +490,4 @@ nsresult SRICheckDataVerifier::ExportEmptyDataSummary(uint32_t aDataLen,
   return NS_OK;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/security/SRICheck.h b/dom/security/SRICheck.h
index 90d26a689226..dca0fde633c9 100644
--- a/dom/security/SRICheck.h
+++ b/dom/security/SRICheck.h
@@ -15,8 +15,7 @@
 class nsIChannel;
 class nsIConsoleReportCollector;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class SRIMetadata;
 
@@ -101,7 +100,6 @@ class SRICheckDataVerifier final {
                       nsIConsoleReportCollector* aReporter);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_SRICheck_h
diff --git a/dom/security/SRILogHelper.h b/dom/security/SRILogHelper.h
index 073a8e801a12..e453f3084275 100644
--- a/dom/security/SRILogHelper.h
+++ b/dom/security/SRILogHelper.h
@@ -9,8 +9,7 @@
 
 #include "mozilla/Logging.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class SRILogHelper final {
  public:
@@ -20,7 +19,6 @@ class SRILogHelper final {
   }
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_SRILogHelper_h
diff --git a/dom/security/SRIMetadata.cpp b/dom/security/SRIMetadata.cpp
index 0f585c4483e2..39097e0de49c 100644
--- a/dom/security/SRIMetadata.cpp
+++ b/dom/security/SRIMetadata.cpp
@@ -20,8 +20,7 @@ static mozilla::LogModule* GetSriMetadataLog() {
 #define SRIMETADATAERROR(args) \
   MOZ_LOG(GetSriMetadataLog(), mozilla::LogLevel::Error, args)
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 SRIMetadata::SRIMetadata(const nsACString& aToken)
     : mAlgorithmType(SRIMetadata::UNKNOWN_ALGORITHM), mEmpty(false) {
@@ -186,5 +185,4 @@ bool SRIMetadata::CanTrustBeDelegatedTo(const SRIMetadata& aOther) const {
   return true;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/security/SRIMetadata.h b/dom/security/SRIMetadata.h
index 37c05ed7b339..a38e92403970 100644
--- a/dom/security/SRIMetadata.h
+++ b/dom/security/SRIMetadata.h
@@ -11,8 +11,7 @@
 #include "nsString.h"
 #include "SRICheck.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class SRIMetadata final {
   friend class SRICheck;
@@ -86,7 +85,6 @@ class SRIMetadata final {
   bool mEmpty;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_SRIMetadata_h
diff --git a/dom/security/SecFetch.h b/dom/security/SecFetch.h
index 9ebec60de532..b4b1495f4dfe 100644
--- a/dom/security/SecFetch.h
+++ b/dom/security/SecFetch.h
@@ -9,8 +9,7 @@
 
 class nsIHttpChannel;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class SecFetch final {
  public:
@@ -23,7 +22,6 @@ class SecFetch final {
   static void AddSecFetchUser(nsIHttpChannel* aHTTPChannel);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_SecFetch_h
diff --git a/dom/security/featurepolicy/Feature.cpp b/dom/security/featurepolicy/Feature.cpp
index 1b9a19862c6c..92a92369da41 100644
--- a/dom/security/featurepolicy/Feature.cpp
+++ b/dom/security/featurepolicy/Feature.cpp
@@ -7,8 +7,7 @@
 #include "Feature.h"
 #include "mozilla/BasePrincipal.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 void Feature::GetAllowList(nsTArray<nsCOMPtr<nsIPrincipal>>& aList) const {
   MOZ_ASSERT(mPolicy == eAllowList);
@@ -74,5 +73,4 @@ bool Feature::AllowListContains(nsIPrincipal* aPrincipal) const {
 
 bool Feature::HasAllowList() const { return mPolicy == eAllowList; }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/security/featurepolicy/Feature.h b/dom/security/featurepolicy/Feature.h
index f85b3addcdf4..e8a3d89c81b6 100644
--- a/dom/security/featurepolicy/Feature.h
+++ b/dom/security/featurepolicy/Feature.h
@@ -13,8 +13,7 @@
 
 class nsIPrincipal;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class Feature final {
  public:
@@ -61,7 +60,6 @@ class Feature final {
   CopyableTArray<nsCOMPtr<nsIPrincipal>> mAllowList;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_Feature_h
diff --git a/dom/security/featurepolicy/FeaturePolicy.cpp b/dom/security/featurepolicy/FeaturePolicy.cpp
index 07b6e9fba3fd..cb5c1ea44a3c 100644
--- a/dom/security/featurepolicy/FeaturePolicy.cpp
+++ b/dom/security/featurepolicy/FeaturePolicy.cpp
@@ -14,8 +14,7 @@
 #include "nsContentUtils.h"
 #include "nsNetUtil.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(FeaturePolicy)
 NS_IMPL_CYCLE_COLLECTING_ADDREF(FeaturePolicy)
@@ -332,5 +331,4 @@ void FeaturePolicy::MaybeSetAllowedPolicy(const nsAString& aFeatureName) {
   mAttributeEnabledFeatureNames.AppendElement(aFeatureName);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom