diff --git a/dom/base/ChromeUtils.cpp b/dom/base/ChromeUtils.cpp
index 7600b9193056..f346c531f0cb 100644
--- a/dom/base/ChromeUtils.cpp
+++ b/dom/base/ChromeUtils.cpp
@@ -23,7 +23,6 @@
 #include "mozilla/ResultExtensions.h"
 #include "mozilla/TimeStamp.h"
 #include "mozilla/dom/BrowsingContext.h"
-#include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/IdleDeadline.h"
 #include "mozilla/dom/JSWindowActorService.h"
@@ -1188,11 +1187,6 @@ void ChromeUtils::GenerateMediaControlKeysTestEvent(
   }
 }
 
-/* static */
-nsIContentChild* ChromeUtils::GetContentChild(const GlobalObject&) {
-  return ContentChild::GetSingleton();
-}
-
 /* static */
 void ChromeUtils::GetCurrentActiveMediaMetadata(const GlobalObject& aGlobal,
                                                 MediaMetadataInit& aMetadata) {
diff --git a/dom/base/ChromeUtils.h b/dom/base/ChromeUtils.h
index b4909fbd6026..d585ce387afb 100644
--- a/dom/base/ChromeUtils.h
+++ b/dom/base/ChromeUtils.h
@@ -11,7 +11,6 @@
 #include "mozilla/dom/BindingDeclarations.h"
 #include "mozilla/dom/ChromeUtilsBinding.h"
 #include "mozilla/ErrorResult.h"
-#include "nsIContentChild.h"
 
 namespace mozilla {
 
@@ -204,8 +203,6 @@ class ChromeUtils {
   static void GenerateMediaControlKeysTestEvent(
       const GlobalObject& aGlobal, MediaControlKeysTestEvent aEvent);
 
-  static nsIContentChild* GetContentChild(const GlobalObject&);
-
   // This function would only be used for testing.
   static void GetCurrentActiveMediaMetadata(const GlobalObject& aGlobal,
                                             MediaMetadataInit& aMetadata);
diff --git a/dom/bindings/Bindings.conf b/dom/bindings/Bindings.conf
index 68582a0e8a57..8eb684484065 100644
--- a/dom/bindings/Bindings.conf
+++ b/dom/bindings/Bindings.conf
@@ -1885,8 +1885,6 @@ addExternalIface('nsITreeSelection', nativeType='nsITreeSelection',
                  notflattened=True)
 addExternalIface('nsISupports', nativeType='nsISupports')
 addExternalIface('nsIDocShell', nativeType='nsIDocShell', notflattened=True)
-addExternalIface('nsIContentChild', nativeType='nsIContentChild', notflattened=True)
-addExternalIface('nsIContentParent', nativeType='nsIContentParent', notflattened=True)
 addExternalIface('nsIReferrerInfo', nativeType='nsIReferrerInfo', notflattened=True)
 addExternalIface('nsIWebNavigation', nativeType='nsIWebNavigation', notflattened=True)
 addExternalIface('nsIEditor', nativeType='nsIEditor', notflattened=True)
diff --git a/dom/chrome-webidl/ChromeUtils.webidl b/dom/chrome-webidl/ChromeUtils.webidl
index a185b94ee794..79c144026a83 100644
--- a/dom/chrome-webidl/ChromeUtils.webidl
+++ b/dom/chrome-webidl/ChromeUtils.webidl
@@ -446,12 +446,6 @@ partial namespace ChromeUtils {
   [ChromeOnly]
   void generateMediaControlKeysTestEvent(MediaControlKeysTestEvent aEvent);
 
-  /**
-   * ContentChild of the current process.
-   */
-  [ChromeOnly]
-  readonly attribute nsIContentChild? contentChild;
-
   // This is used to get the media metadata from the current main controller in
   // testing.
   [ChromeOnly]
diff --git a/dom/chrome-webidl/JSWindowActor.webidl b/dom/chrome-webidl/JSWindowActor.webidl
index c330133b35f3..403acf98b996 100644
--- a/dom/chrome-webidl/JSWindowActor.webidl
+++ b/dom/chrome-webidl/JSWindowActor.webidl
@@ -5,8 +5,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 interface nsISupports;
-interface nsIContentChild;
-interface nsIContentParent;
 
 interface mixin JSWindowActor {
   [Throws]
diff --git a/dom/chrome-webidl/WindowGlobalActors.webidl b/dom/chrome-webidl/WindowGlobalActors.webidl
index 25dd41eabc40..1a77444d1b9f 100644
--- a/dom/chrome-webidl/WindowGlobalActors.webidl
+++ b/dom/chrome-webidl/WindowGlobalActors.webidl
@@ -54,10 +54,6 @@ interface WindowGlobalParent : WindowContext {
   // String containing serialized content blocking log.
   readonly attribute DOMString contentBlockingLog;
 
-  // ContentParent of the process this window is loaded in.
-  // Will be `null` for windows loaded in the parent process.
-  readonly attribute nsIContentParent? contentParent;
-
   static WindowGlobalParent? getByInnerWindowId(unsigned long long innerWindowId);
 
   /**
diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index d2c346478279..4330f7edb690 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -604,9 +604,8 @@ ContentChild::~ContentChild() {
 #endif
 
 NS_INTERFACE_MAP_BEGIN(ContentChild)
-  NS_INTERFACE_MAP_ENTRY(nsIContentChild)
   NS_INTERFACE_MAP_ENTRY(nsIWindowProvider)
-  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIContentChild)
+  NS_INTERFACE_MAP_ENTRY(nsISupports)
 NS_INTERFACE_MAP_END
 
 mozilla::ipc::IPCResult ContentChild::RecvSetXPCOMProcessAttributes(
@@ -2871,8 +2870,7 @@ void ContentChild::ForceKillTimerCallback(nsITimer* aTimer, void* aClosure) {
 mozilla::ipc::IPCResult ContentChild::RecvShutdown() {
   nsCOMPtr<nsIObserverService> os = services::GetObserverService();
   if (os) {
-    os->NotifyObservers(ToSupports(this), "content-child-will-shutdown",
-                        nullptr);
+    os->NotifyObservers(this, "content-child-will-shutdown", nullptr);
   }
 
   ShutdownInternal();
@@ -2919,7 +2917,7 @@ void ContentChild::ShutdownInternal() {
 
   nsCOMPtr<nsIObserverService> os = services::GetObserverService();
   if (os) {
-    os->NotifyObservers(ToSupports(this), "content-child-shutdown", nullptr);
+    os->NotifyObservers(this, "content-child-shutdown", nullptr);
   }
 
 #if defined(XP_WIN)
@@ -4288,11 +4286,6 @@ mozilla::ipc::IPCResult ContentChild::RecvInitSandboxTesting(
 }
 #endif
 
-NS_IMETHODIMP ContentChild::GetChildID(uint64_t* aOut) {
-  *aOut = mID;
-  return NS_OK;
-}
-
 }  // namespace dom
 
 #if defined(__OpenBSD__) && defined(MOZ_SANDBOX)
diff --git a/dom/ipc/ContentChild.h b/dom/ipc/ContentChild.h
index 5e174dfc3b96..c59d04cd650b 100644
--- a/dom/ipc/ContentChild.h
+++ b/dom/ipc/ContentChild.h
@@ -21,7 +21,6 @@
 #include "mozilla/jsipc/CrossProcessObjectWrappers.h"
 #include "nsAutoPtr.h"
 #include "nsHashKeys.h"
-#include "nsIContentChild.h"
 #include "nsIObserver.h"
 #include "nsTHashtable.h"
 #include "nsStringFwd.h"
@@ -80,7 +79,6 @@ enum class MediaControlKeysEvent : uint32_t;
 
 class ContentChild final
     : public PContentChild,
-      public nsIContentChild,
       public nsIWindowProvider,
       public CPOWManagerGetter,
       public mozilla::ipc::IShmemAllocator,
@@ -93,7 +91,6 @@ class ContentChild final
   friend class PContentChild;
 
  public:
-  NS_DECL_NSICONTENTCHILD
   NS_DECL_NSIWINDOWPROVIDER
 
   ContentChild();
@@ -897,10 +894,6 @@ class ContentChild final
   DISALLOW_EVIL_CONSTRUCTORS(ContentChild);
 };
 
-inline nsISupports* ToSupports(mozilla::dom::ContentChild* aContentChild) {
-  return static_cast<nsIContentChild*>(aContentChild);
-}
-
 }  // namespace dom
 }  // namespace mozilla
 
diff --git a/dom/ipc/ContentParent.cpp b/dom/ipc/ContentParent.cpp
index 32d494f86185..2c3fda27c080 100644
--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -2922,12 +2922,11 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(ContentParent)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ContentParent)
   NS_INTERFACE_MAP_ENTRY_CONCRETE(ContentParent)
-  NS_INTERFACE_MAP_ENTRY(nsIContentParent)
   NS_INTERFACE_MAP_ENTRY(nsIObserver)
   NS_INTERFACE_MAP_ENTRY(nsIDOMGeoPositionCallback)
   NS_INTERFACE_MAP_ENTRY(nsIDOMGeoPositionErrorCallback)
   NS_INTERFACE_MAP_ENTRY(nsIInterfaceRequestor)
-  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIContentParent)
+  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIObserver)
 NS_INTERFACE_MAP_END
 
 NS_IMETHODIMP
@@ -6453,11 +6452,6 @@ mozilla::ipc::IPCResult ContentParent::RecvCommitWindowContextTransaction(
   return aTransaction.CommitFromIPC(aContext, this);
 }
 
-NS_IMETHODIMP ContentParent::GetChildID(uint64_t* aOut) {
-  *aOut = this->ChildID();
-  return NS_OK;
-}
-
 }  // namespace dom
 }  // namespace mozilla
 
diff --git a/dom/ipc/ContentParent.h b/dom/ipc/ContentParent.h
index c1c6bd80f6f2..34859208babd 100644
--- a/dom/ipc/ContentParent.h
+++ b/dom/ipc/ContentParent.h
@@ -35,7 +35,6 @@
 #include "nsPluginTags.h"
 #include "nsFrameMessageManager.h"
 #include "nsHashKeys.h"
-#include "nsIContentParent.h"
 #include "nsIInterfaceRequestor.h"
 #include "nsIObserver.h"
 #include "nsIRemoteTab.h"
@@ -133,7 +132,6 @@ struct CancelContentJSOptions;
 
 class ContentParent final
     : public PContentParent,
-      public nsIContentParent,
       public nsIObserver,
       public nsIDOMGeoPositionCallback,
       public nsIDOMGeoPositionErrorCallback,
@@ -1302,8 +1300,6 @@ class ContentParent final
 
   static bool ShouldSyncPreference(const char16_t* aData);
 
-  NS_IMETHOD GetChildID(uint64_t* aChildID) override;
-
  private:
   // Return an existing ContentParent if possible. Otherwise, `nullptr`.
   static already_AddRefed<ContentParent> GetUsedBrowserProcess(
@@ -1495,10 +1491,6 @@ class RemoteWindowContext final : public nsIRemoteWindowContext,
   RefPtr<BrowserParent> mBrowserParent;
 };
 
-inline nsISupports* ToSupports(mozilla::dom::ContentParent* aContentParent) {
-  return static_cast<nsIContentParent*>(aContentParent);
-}
-
 }  // namespace dom
 }  // namespace mozilla
 
diff --git a/dom/ipc/JSWindowActorChild.h b/dom/ipc/JSWindowActorChild.h
index fd38d42dda9e..2a2e883bd602 100644
--- a/dom/ipc/JSWindowActorChild.h
+++ b/dom/ipc/JSWindowActorChild.h
@@ -11,11 +11,9 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BindingDeclarations.h"
-#include "mozilla/dom/ContentChild.h"
 #include "mozilla/dom/JSWindowActor.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsWrapperCache.h"
-#include "nsIContentChild.h"
 
 namespace mozilla {
 namespace dom {
diff --git a/dom/ipc/JSWindowActorParent.cpp b/dom/ipc/JSWindowActorParent.cpp
index f6cec04ffd14..86db3131565b 100644
--- a/dom/ipc/JSWindowActorParent.cpp
+++ b/dom/ipc/JSWindowActorParent.cpp
@@ -6,7 +6,6 @@
 
 #include "mozilla/dom/JSWindowActorBinding.h"
 #include "mozilla/dom/JSWindowActorParent.h"
-#include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/WindowGlobalParent.h"
 #include "mozilla/dom/MessageManagerBinding.h"
 
diff --git a/dom/ipc/JSWindowActorParent.h b/dom/ipc/JSWindowActorParent.h
index dfeab3a3f492..c5bf0db2263c 100644
--- a/dom/ipc/JSWindowActorParent.h
+++ b/dom/ipc/JSWindowActorParent.h
@@ -11,7 +11,6 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/dom/BindingDeclarations.h"
-#include "mozilla/dom/ContentParent.h"
 #include "mozilla/dom/JSWindowActor.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsWrapperCache.h"
diff --git a/dom/ipc/WindowGlobalParent.cpp b/dom/ipc/WindowGlobalParent.cpp
index 4d80cf19cd15..3ee0ecd02674 100644
--- a/dom/ipc/WindowGlobalParent.cpp
+++ b/dom/ipc/WindowGlobalParent.cpp
@@ -649,15 +649,6 @@ nsIGlobalObject* WindowGlobalParent::GetParentObject() {
   return xpc::NativeGlobal(xpc::PrivilegedJunkScope());
 }
 
-ContentParent* WindowGlobalParent::GetContentParent() {
-  RefPtr<BrowserParent> browserParent = GetBrowserParent();
-  if (!browserParent) {
-    return nullptr;
-  }
-
-  return browserParent->Manager();
-}
-
 NS_IMPL_CYCLE_COLLECTION_INHERITED(WindowGlobalParent, WindowContext,
                                    mWindowActors)
 
diff --git a/dom/ipc/WindowGlobalParent.h b/dom/ipc/WindowGlobalParent.h
index b63a779c3936..deae5984f646 100644
--- a/dom/ipc/WindowGlobalParent.h
+++ b/dom/ipc/WindowGlobalParent.h
@@ -19,7 +19,6 @@
 #include "nsRefPtrHashtable.h"
 #include "nsWrapperCache.h"
 #include "nsISupports.h"
-#include "nsIContentParent.h"
 #include "mozilla/dom/WindowGlobalActor.h"
 #include "mozilla/dom/CanonicalBrowsingContext.h"
 
@@ -156,8 +155,6 @@ class WindowGlobalParent final : public WindowContext,
 
   ContentBlockingLog* GetContentBlockingLog() { return &mContentBlockingLog; }
 
-  ContentParent* GetContentParent();
-
  protected:
   const nsAString& GetRemoteType() override;
   JSWindowActor::Type GetSide() override { return JSWindowActor::Type::Parent; }
diff --git a/dom/ipc/moz.build b/dom/ipc/moz.build
index 93ea3846b7d2..eabd6bc58ebd 100644
--- a/dom/ipc/moz.build
+++ b/dom/ipc/moz.build
@@ -8,8 +8,6 @@ with Files("**"):
     BUG_COMPONENT = ("Core", "DOM: Content Processes")
 
 XPIDL_SOURCES += [
-    'nsIContentChild.idl',
-    'nsIContentParent.idl',
     'nsIHangReport.idl',
     'nsIRemoteWebProgress.idl',
     'nsIRemoteWebProgressRequest.idl',
diff --git a/dom/ipc/nsIContentChild.idl b/dom/ipc/nsIContentChild.idl
deleted file mode 100644
index d74bfb73d00b..000000000000
--- a/dom/ipc/nsIContentChild.idl
+++ /dev/null
@@ -1,7 +0,0 @@
-#include "nsISupports.idl"
-
-[scriptable, builtinclass, uuid(b0c6e5f3-02f1-4f11-a0af-336fc231f3bf)]
-interface nsIContentChild: nsISupports {
-    // Internal ID of the process.
-    readonly attribute unsigned long long childID;
-};
diff --git a/dom/ipc/nsIContentParent.idl b/dom/ipc/nsIContentParent.idl
deleted file mode 100644
index bd4d0ebdaa6d..000000000000
--- a/dom/ipc/nsIContentParent.idl
+++ /dev/null
@@ -1,7 +0,0 @@
-#include "nsISupports.idl"
-
-[scriptable, builtinclass, uuid(81fc08b9-c901-471f-ab0d-876362eba770)]
-interface nsIContentParent: nsISupports {
-    // Internal ID of the process.
-    readonly attribute unsigned long long childID;
-};
diff --git a/dom/ipc/tests/JSWindowActor/browser.ini b/dom/ipc/tests/JSWindowActor/browser.ini
index 5c23349037fe..fe8073def403 100644
--- a/dom/ipc/tests/JSWindowActor/browser.ini
+++ b/dom/ipc/tests/JSWindowActor/browser.ini
@@ -10,7 +10,6 @@ support-files =
 support-files=
   file_mediaPlayback.html
   audio.ogg
-[browser_process_childid.js]
 [browser_registerWindowActor.js]
 [browser_sendAsyncMessage.js]
 [browser_sendQuery.js]
diff --git a/dom/ipc/tests/JSWindowActor/browser_process_childid.js b/dom/ipc/tests/JSWindowActor/browser_process_childid.js
deleted file mode 100644
index 076419ba0bb5..000000000000
--- a/dom/ipc/tests/JSWindowActor/browser_process_childid.js
+++ /dev/null
@@ -1,27 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-"use strict";
-
-// Test that `process.childID` is defined.
-
-declTest("test childid", {
-  async test(browser) {
-    let parent = browser.browsingContext.currentWindowGlobal;
-    ok(
-      parent.contentParent.childID,
-      "parent contentParent.childID should have a value."
-    );
-    await SpecialPowers.spawn(
-      browser,
-      [parent.contentParent.childID],
-      async function(parentChildID) {
-        ok(
-          ChromeUtils.contentChild.childID,
-          "child process.childID should have a value."
-        );
-        let childID = ChromeUtils.contentChild.childID;
-        is(parentChildID, childID);
-      }
-    );
-  },
-});