diff --git a/layout/style/MediaQueryList.h b/layout/style/MediaQueryList.h
index 9fd1b1ca194a..4d06e6d0b496 100644
--- a/layout/style/MediaQueryList.h
+++ b/layout/style/MediaQueryList.h
@@ -19,8 +19,7 @@
 #include "mozilla/DOMEventTargetHelper.h"
 #include "mozilla/dom/MediaQueryListBinding.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class MediaList;
 
@@ -97,7 +96,6 @@ class MediaQueryList final : public DOMEventTargetHelper,
   bool mMatchesValid;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif /* !defined(mozilla_dom_MediaQueryList_h) */
diff --git a/toolkit/components/sessionstore/SessionStoreListener.h b/toolkit/components/sessionstore/SessionStoreListener.h
index acedceb04b62..2cd35a00f5fd 100644
--- a/toolkit/components/sessionstore/SessionStoreListener.h
+++ b/toolkit/components/sessionstore/SessionStoreListener.h
@@ -16,8 +16,7 @@
 
 class nsITimer;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class ContentSessionStore {
  public:
@@ -110,7 +109,6 @@ class TabListener : public nsIDOMEventListener,
   uint32_t mEpoch;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_SessionStoreListener_h
diff --git a/toolkit/components/sessionstore/SessionStoreRestoreData.h b/toolkit/components/sessionstore/SessionStoreRestoreData.h
index 773227bda161..d88b0f5b4223 100644
--- a/toolkit/components/sessionstore/SessionStoreRestoreData.h
+++ b/toolkit/components/sessionstore/SessionStoreRestoreData.h
@@ -12,8 +12,7 @@
 #include "mozilla/dom/SessionStoreData.h"
 #include "nsISessionStoreRestoreData.h"
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 #define NS_SESSIONSTORERESTOREDATA_IID               \
   {                                                  \
@@ -56,7 +55,6 @@ class SessionStoreRestoreData final : public nsISessionStoreRestoreData {
 NS_DEFINE_STATIC_IID_ACCESSOR(SessionStoreRestoreData,
                               NS_SESSIONSTORERESTOREDATA_IID)
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif
diff --git a/uriloader/exthandler/ExternalHelperAppChild.h b/uriloader/exthandler/ExternalHelperAppChild.h
index 228d7e45b3ad..2b189499fe28 100644
--- a/uriloader/exthandler/ExternalHelperAppChild.h
+++ b/uriloader/exthandler/ExternalHelperAppChild.h
@@ -13,8 +13,7 @@
 
 class nsIDivertableChannel;
 
-namespace mozilla {
-namespace dom {
+namespace mozilla::dom {
 
 class BrowserChild;
 
@@ -40,7 +39,6 @@ class ExternalHelperAppChild : public PExternalHelperAppChild,
   nsresult mStatus;
 };
 
-}  // namespace dom
-}  // namespace mozilla
+}  // namespace mozilla::dom
 
 #endif  // mozilla_dom_ExternalHelperAppChild_h