diff --git a/dom/serviceworkers/ServiceWorkerCloneData.cpp b/dom/serviceworkers/ServiceWorkerCloneData.cpp
index babc07f324f2..e96886edf542 100644
--- a/dom/serviceworkers/ServiceWorkerCloneData.cpp
+++ b/dom/serviceworkers/ServiceWorkerCloneData.cpp
@@ -14,8 +14,7 @@
 #include "nsProxyRelease.h"
 #include "nsThreadUtils.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 ServiceWorkerCloneData::~ServiceWorkerCloneData() {
   RefPtr<ipc::SharedJSAllocatedData> sharedData = TakeSharedData();
@@ -122,5 +121,4 @@ bool ServiceWorkerCloneData::IsErrorMessageData() const {
   return mIsErrorMessageData;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerContainer.cpp b/dom/serviceworkers/ServiceWorkerContainer.cpp
index 8ef3c51caab1..e1d74e5e55b9 100644
--- a/dom/serviceworkers/ServiceWorkerContainer.cpp
+++ b/dom/serviceworkers/ServiceWorkerContainer.cpp
@@ -48,8 +48,7 @@
 #  undef DispatchMessage
 #endif
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ServiceWorkerContainer)
 NS_INTERFACE_MAP_END_INHERITING(DOMEventTargetHelper)
@@ -804,5 +803,4 @@ Result<Ok, bool> ServiceWorkerContainer::FillInMessageEventInit(
   return Ok();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerContainer.h b/dom/serviceworkers/ServiceWorkerContainer.h
index d3ac715734c6..d256db87ad97 100644
--- a/dom/serviceworkers/ServiceWorkerContainer.h
+++ b/dom/serviceworkers/ServiceWorkerContainer.h
@@ -13,8 +13,7 @@
 
 class nsIGlobalWindow;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ClientPostMessageArgs;
 struct MessageEventInit;
@@ -166,7 +165,6 @@ class ServiceWorkerContainer final : public DOMEventTargetHelper {
   nsTArray<RefPtr<ReceivedMessage>> mPendingMessages;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif /* mozilla_dom_serviceworkercontainer_h__ */
diff --git a/dom/serviceworkers/ServiceWorkerContainerChild.cpp b/dom/serviceworkers/ServiceWorkerContainerChild.cpp
index 93e0c2f7ebe8..a4f58ce7ceac 100644
--- a/dom/serviceworkers/ServiceWorkerContainerChild.cpp
+++ b/dom/serviceworkers/ServiceWorkerContainerChild.cpp
@@ -11,8 +11,7 @@
 #include "RemoteServiceWorkerContainerImpl.h"
 #include "ServiceWorkerContainerChild.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 void ServiceWorkerContainerChild::ActorDestroy(ActorDestroyReason aReason) {
   mIPCWorkerRef = nullptr;
@@ -70,5 +69,4 @@ void ServiceWorkerContainerChild::MaybeStartTeardown() {
   mTeardownStarted = true;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerContainerChild.h b/dom/serviceworkers/ServiceWorkerContainerChild.h
index 9c7b74849767..fd71b4ae0d2f 100644
--- a/dom/serviceworkers/ServiceWorkerContainerChild.h
+++ b/dom/serviceworkers/ServiceWorkerContainerChild.h
@@ -12,8 +12,7 @@
 // XXX Avoid including this here by moving function bodies to the cpp file
 #include "mozilla/dom/WorkerRef.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class RemoteServiceWorkerContainerImpl;
 
@@ -43,7 +42,6 @@ class ServiceWorkerContainerChild final : public PServiceWorkerContainerChild {
   void MaybeStartTeardown();
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkercontainerchild_h__
diff --git a/dom/serviceworkers/ServiceWorkerContainerParent.cpp b/dom/serviceworkers/ServiceWorkerContainerParent.cpp
index d00caf3f0ad1..f449d03980a2 100644
--- a/dom/serviceworkers/ServiceWorkerContainerParent.cpp
+++ b/dom/serviceworkers/ServiceWorkerContainerParent.cpp
@@ -9,8 +9,7 @@
 #include "ServiceWorkerContainerProxy.h"
 #include "mozilla/dom/ClientInfo.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 using mozilla::ipc::IPCResult;
 
@@ -127,5 +126,4 @@ void ServiceWorkerContainerParent::Init() {
   mProxy = new ServiceWorkerContainerProxy(this);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerContainerParent.h b/dom/serviceworkers/ServiceWorkerContainerParent.h
index ccfdced8f649..e794863542c8 100644
--- a/dom/serviceworkers/ServiceWorkerContainerParent.h
+++ b/dom/serviceworkers/ServiceWorkerContainerParent.h
@@ -9,8 +9,7 @@
 
 #include "mozilla/dom/PServiceWorkerContainerParent.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class IPCServiceWorkerDescriptor;
 class ServiceWorkerContainerProxy;
@@ -51,7 +50,6 @@ class ServiceWorkerContainerParent final
   void Init();
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkercontainerparent_h__
diff --git a/dom/serviceworkers/ServiceWorkerContainerProxy.cpp b/dom/serviceworkers/ServiceWorkerContainerProxy.cpp
index 9574e062e378..b6192091cdec 100644
--- a/dom/serviceworkers/ServiceWorkerContainerProxy.cpp
+++ b/dom/serviceworkers/ServiceWorkerContainerProxy.cpp
@@ -12,8 +12,7 @@
 #include "mozilla/SchedulerGroup.h"
 #include "mozilla/ScopeExit.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 using mozilla::ipc::AssertIsOnBackgroundThread;
 
@@ -150,5 +149,4 @@ RefPtr<ServiceWorkerRegistrationPromise> ServiceWorkerContainerProxy::GetReady(
   return promise;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerContainerProxy.h b/dom/serviceworkers/ServiceWorkerContainerProxy.h
index 7eaee6fbacec..06019aefb857 100644
--- a/dom/serviceworkers/ServiceWorkerContainerProxy.h
+++ b/dom/serviceworkers/ServiceWorkerContainerProxy.h
@@ -10,8 +10,7 @@
 #include "mozilla/dom/ServiceWorkerUtils.h"
 #include "mozilla/RefPtr.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ServiceWorkerContainerParent;
 
@@ -42,7 +41,6 @@ class ServiceWorkerContainerProxy final {
   NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ServiceWorkerContainerProxy);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // moz_dom_ServiceWorkerContainerProxy_h
diff --git a/dom/serviceworkers/ServiceWorkerDescriptor.cpp b/dom/serviceworkers/ServiceWorkerDescriptor.cpp
index a418623f566a..898f271feaf3 100644
--- a/dom/serviceworkers/ServiceWorkerDescriptor.cpp
+++ b/dom/serviceworkers/ServiceWorkerDescriptor.cpp
@@ -9,8 +9,7 @@
 #include "mozilla/dom/ServiceWorkerBinding.h"
 #include "mozilla/ipc/PBackgroundSharedTypes.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 using mozilla::ipc::PrincipalInfo;
 using mozilla::ipc::PrincipalInfoToPrincipal;
@@ -139,5 +138,4 @@ const IPCServiceWorkerDescriptor& ServiceWorkerDescriptor::ToIPC() const {
   return *mData;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerEvents.cpp b/dom/serviceworkers/ServiceWorkerEvents.cpp
index efee446630e5..95eb52a95cca 100644
--- a/dom/serviceworkers/ServiceWorkerEvents.cpp
+++ b/dom/serviceworkers/ServiceWorkerEvents.cpp
@@ -89,8 +89,7 @@ void AsyncLog(nsIInterceptedChannel* aInterceptedChannel,
 
 }  // anonymous namespace
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 CancelChannelRunnable::CancelChannelRunnable(
     nsMainThreadPtrHandle<nsIInterceptedChannel>& aChannel,
@@ -1276,5 +1275,4 @@ NS_INTERFACE_MAP_END_INHERITING(Event)
 NS_IMPL_ADDREF_INHERITED(ExtendableMessageEvent, Event)
 NS_IMPL_RELEASE_INHERITED(ExtendableMessageEvent, Event)
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerEvents.h b/dom/serviceworkers/ServiceWorkerEvents.h
index 63fe283edb33..7cbb6d9bcd43 100644
--- a/dom/serviceworkers/ServiceWorkerEvents.h
+++ b/dom/serviceworkers/ServiceWorkerEvents.h
@@ -22,8 +22,7 @@
 
 class nsIInterceptedChannel;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class Blob;
 class Client;
@@ -305,7 +304,6 @@ class ExtendableMessageEvent final : public ExtendableEvent {
   void GetPorts(nsTArray<RefPtr<MessagePort>>& aPorts);
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif /* mozilla_dom_serviceworkerevents_h__ */
diff --git a/dom/serviceworkers/ServiceWorkerInfo.cpp b/dom/serviceworkers/ServiceWorkerInfo.cpp
index edd82be6b857..9998cfed6bc1 100644
--- a/dom/serviceworkers/ServiceWorkerInfo.cpp
+++ b/dom/serviceworkers/ServiceWorkerInfo.cpp
@@ -14,8 +14,7 @@
 #include "mozilla/dom/RemoteWorkerTypes.h"
 #include "mozilla/dom/WorkerPrivate.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 using mozilla::ipc::PrincipalInfo;
 
@@ -284,5 +283,4 @@ void ServiceWorkerInfo::SetRegistrationVersion(uint64_t aVersion) {
   mDescriptor.SetRegistrationVersion(aVersion);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerInfo.h b/dom/serviceworkers/ServiceWorkerInfo.h
index 040a69dfa8bc..03a9eb6affca 100644
--- a/dom/serviceworkers/ServiceWorkerInfo.h
+++ b/dom/serviceworkers/ServiceWorkerInfo.h
@@ -15,8 +15,7 @@
 #include "mozilla/TimeStamp.h"
 #include "nsIServiceWorkerManager.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ClientInfoAndState;
 class ClientState;
@@ -179,7 +178,6 @@ class ServiceWorkerInfo final : public nsIServiceWorkerInfo {
   }
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkerinfo_h
diff --git a/dom/serviceworkers/ServiceWorkerInterceptController.cpp b/dom/serviceworkers/ServiceWorkerInterceptController.cpp
index 065811e75eb5..41bf644f65d1 100644
--- a/dom/serviceworkers/ServiceWorkerInterceptController.cpp
+++ b/dom/serviceworkers/ServiceWorkerInterceptController.cpp
@@ -18,8 +18,7 @@
 #include "ServiceWorkerManager.h"
 #include "nsIPrincipal.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 NS_IMPL_ISUPPORTS(ServiceWorkerInterceptController,
                   nsINetworkInterceptController)
@@ -129,5 +128,4 @@ ServiceWorkerInterceptController::ChannelIntercepted(
   return NS_OK;
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerInterceptController.h b/dom/serviceworkers/ServiceWorkerInterceptController.h
index 43f24dc776d7..32113291efc6 100644
--- a/dom/serviceworkers/ServiceWorkerInterceptController.h
+++ b/dom/serviceworkers/ServiceWorkerInterceptController.h
@@ -9,8 +9,7 @@
 
 #include "nsINetworkInterceptController.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ServiceWorkerInterceptController final
     : public nsINetworkInterceptController {
@@ -21,7 +20,6 @@ class ServiceWorkerInterceptController final
   NS_DECL_NSINETWORKINTERCEPTCONTROLLER
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkerinterceptcontroller_h
diff --git a/dom/serviceworkers/ServiceWorkerJob.cpp b/dom/serviceworkers/ServiceWorkerJob.cpp
index 7e73d7b7dd01..980d48b66ca1 100644
--- a/dom/serviceworkers/ServiceWorkerJob.cpp
+++ b/dom/serviceworkers/ServiceWorkerJob.cpp
@@ -12,8 +12,7 @@
 #include "nsThreadUtils.h"
 #include "ServiceWorkerManager.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 ServiceWorkerJob::Type ServiceWorkerJob::GetType() const { return mType; }
 
@@ -218,5 +217,4 @@ void ServiceWorkerJob::Finish(nsresult aRv) {
   Finish(converted);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerJobQueue.cpp b/dom/serviceworkers/ServiceWorkerJobQueue.cpp
index db7b2748a125..497265249a37 100644
--- a/dom/serviceworkers/ServiceWorkerJobQueue.cpp
+++ b/dom/serviceworkers/ServiceWorkerJobQueue.cpp
@@ -10,8 +10,7 @@
 #include "ServiceWorkerJob.h"
 #include "mozilla/dom/WorkerCommon.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ServiceWorkerJobQueue::Callback final
     : public ServiceWorkerJob::Callback {
@@ -118,5 +117,4 @@ void ServiceWorkerJobQueue::CancelAll() {
   }
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerJobQueue.h b/dom/serviceworkers/ServiceWorkerJobQueue.h
index 09be22a09139..9e51aefe4f21 100644
--- a/dom/serviceworkers/ServiceWorkerJobQueue.h
+++ b/dom/serviceworkers/ServiceWorkerJobQueue.h
@@ -10,8 +10,7 @@
 #include "mozilla/RefPtr.h"
 #include "nsTArray.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ServiceWorkerJob;
 
@@ -36,7 +35,6 @@ class ServiceWorkerJobQueue final {
   NS_INLINE_DECL_REFCOUNTING(ServiceWorkerJobQueue)
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkerjobqueue_h
diff --git a/dom/serviceworkers/ServiceWorkerManager.cpp b/dom/serviceworkers/ServiceWorkerManager.cpp
index 2cb58210e3fa..a0579486c203 100644
--- a/dom/serviceworkers/ServiceWorkerManager.cpp
+++ b/dom/serviceworkers/ServiceWorkerManager.cpp
@@ -108,8 +108,7 @@ using namespace mozilla;
 using namespace mozilla::dom;
 using namespace mozilla::ipc;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 // Counts the number of registered ServiceWorkers, and the number that
 // handle Fetch, for reporting in Telemetry
@@ -3384,5 +3383,4 @@ void ServiceWorkerManager::ReportServiceWorkerShutdownProgress(
   mShutdownBlocker->ReportShutdownProgress(aShutdownStateId, aProgress);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerOp.cpp b/dom/serviceworkers/ServiceWorkerOp.cpp
index eb450c2cc21d..9513abcbaa5e 100644
--- a/dom/serviceworkers/ServiceWorkerOp.cpp
+++ b/dom/serviceworkers/ServiceWorkerOp.cpp
@@ -65,8 +65,7 @@
 #include "mozilla/ipc/IPCStreamUtils.h"
 #include "mozilla/net/MozURL.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 namespace {
 
@@ -1897,5 +1896,4 @@ class ExtensionAPIEventOp final : public ServiceWorkerOp {
   return op.forget();
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerOp.h b/dom/serviceworkers/ServiceWorkerOp.h
index 3f7bedf13526..46aee832dc30 100644
--- a/dom/serviceworkers/ServiceWorkerOp.h
+++ b/dom/serviceworkers/ServiceWorkerOp.h
@@ -23,8 +23,7 @@
 #include "mozilla/dom/ServiceWorkerOpArgs.h"
 #include "mozilla/dom/WorkerRunnable.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class FetchEventOpProxyChild;
 
@@ -189,7 +188,6 @@ class FetchEventOp final : public ExtendableEventOp,
   TimeStamp mFetchHandlerFinish;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkerop_h__
diff --git a/dom/serviceworkers/ServiceWorkerOpPromise.h b/dom/serviceworkers/ServiceWorkerOpPromise.h
index bb8df8bdc740..b2271434a0a6 100644
--- a/dom/serviceworkers/ServiceWorkerOpPromise.h
+++ b/dom/serviceworkers/ServiceWorkerOpPromise.h
@@ -14,8 +14,7 @@
 #include "mozilla/dom/SafeRefPtr.h"
 #include "mozilla/dom/ServiceWorkerOpArgs.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class InternalResponse;
 
@@ -45,7 +44,6 @@ using ServiceWorkerOpPromise =
 using ServiceWorkerFetchEventOpPromise =
     MozPromise<ServiceWorkerFetchEventOpResult, nsresult, true>;
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkeroppromise_h__
diff --git a/dom/serviceworkers/ServiceWorkerParent.cpp b/dom/serviceworkers/ServiceWorkerParent.cpp
index 87d81987e0a9..f21c5711fe64 100644
--- a/dom/serviceworkers/ServiceWorkerParent.cpp
+++ b/dom/serviceworkers/ServiceWorkerParent.cpp
@@ -12,8 +12,7 @@
 #include "mozilla/dom/ClientState.h"
 #include "mozilla/dom/ipc/StructuredCloneData.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 using mozilla::dom::ipc::StructuredCloneData;
 using mozilla::ipc::IPCResult;
@@ -60,5 +59,4 @@ void ServiceWorkerParent::MaybeSendDelete() {
   Unused << Send__delete__(this);
 }
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
diff --git a/dom/serviceworkers/ServiceWorkerParent.h b/dom/serviceworkers/ServiceWorkerParent.h
index f3a4138cbfb7..7224c632c7b7 100644
--- a/dom/serviceworkers/ServiceWorkerParent.h
+++ b/dom/serviceworkers/ServiceWorkerParent.h
@@ -9,8 +9,7 @@
 
 #include "mozilla/dom/PServiceWorkerParent.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class IPCServiceWorkerDescriptor;
 class ServiceWorkerProxy;
@@ -40,7 +39,6 @@ class ServiceWorkerParent final : public PServiceWorkerParent {
   void MaybeSendDelete();
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_serviceworkerparent_h__