--- a/chrome/nsChromeRegistry.cpp
+++ b/chrome/nsChromeRegistry.cpp
@@ -105,19 +105,16 @@ nsChromeRegistry::~nsChromeRegistry()
 {
   gChromeRegistry = nullptr;
 }
 
 NS_INTERFACE_MAP_BEGIN(nsChromeRegistry)
   NS_INTERFACE_MAP_ENTRY(nsIChromeRegistry)
   NS_INTERFACE_MAP_ENTRY(nsIXULChromeRegistry)
   NS_INTERFACE_MAP_ENTRY(nsIToolkitChromeRegistry)
-#ifdef MOZ_XUL
-  NS_INTERFACE_MAP_ENTRY(nsIXULOverlayProvider)
-#endif
   NS_INTERFACE_MAP_ENTRY(nsIObserver)
   NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
   NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIChromeRegistry)
 NS_INTERFACE_MAP_END
 
 NS_IMPL_ADDREF(nsChromeRegistry)
 NS_IMPL_RELEASE(nsChromeRegistry)
 

--- a/chrome/nsChromeRegistry.h
+++ b/chrome/nsChromeRegistry.h
@@ -5,20 +5,16 @@
 
 #ifndef nsChromeRegistry_h
 #define nsChromeRegistry_h
 
 #include "nsIToolkitChromeRegistry.h"
 #include "nsIObserver.h"
 #include "nsWeakReference.h"
 
-#ifdef MOZ_XUL
-#include "nsIXULOverlayProvider.h"
-#endif
-
 #include "nsString.h"
 #include "nsURIHashKey.h"
 #include "nsInterfaceHashtable.h"
 #include "nsXULAppAPI.h"
 
 #include "mozilla/FileLocation.h"
 
 class nsPIDOMWindowOuter;
@@ -31,19 +27,16 @@ class nsIURL;
 // sense for one side erroring out in the other.
 
 // for component registration
 // {47049e42-1d87-482a-984d-56ae185e367a}
 #define NS_CHROMEREGISTRY_CID \
 { 0x47049e42, 0x1d87, 0x482a, { 0x98, 0x4d, 0x56, 0xae, 0x18, 0x5e, 0x36, 0x7a } }
 
 class nsChromeRegistry : public nsIToolkitChromeRegistry,
-#ifdef MOZ_XUL
-                         public nsIXULOverlayProvider,
-#endif
                          public nsIObserver,
                          public nsSupportsWeakReference
 {
 public:
   NS_DECL_ISUPPORTS
 
   // nsIXULChromeRegistry methods:
   NS_IMETHOD ReloadChrome() override;

--- a/chrome/nsChromeRegistryChrome.cpp
+++ b/chrome/nsChromeRegistryChrome.cpp
@@ -569,33 +569,16 @@ nsChromeRegistryChrome::OverlayListHash:
 {
   OverlayListEntry* entry = mTable.GetEntry(aBase);
   if (!entry)
     return nullptr;
 
   return &entry->mArray;
 }
 
-#ifdef MOZ_XUL
-NS_IMETHODIMP
-nsChromeRegistryChrome::GetXULOverlays(nsIURI *aChromeURL,
-                                       nsISimpleEnumerator **aResult)
-{
-  nsCOMPtr<nsIURI> chromeURLWithoutHash;
-  if (aChromeURL) {
-    aChromeURL->CloneIgnoringRef(getter_AddRefs(chromeURLWithoutHash));
-  }
-  const nsCOMArray<nsIURI>* parray = mOverlayHash.GetArray(chromeURLWithoutHash);
-  if (!parray)
-    return NS_NewEmptyEnumerator(aResult);
-
-  return NS_NewArrayEnumerator(aResult, *parray);
-}
-#endif // MOZ_XUL
-
 nsIURI*
 nsChromeRegistry::ManifestProcessingContext::GetManifestURI()
 {
   if (!mManifestURI) {
     nsCString uri;
     mFile.GetURIString(uri);
     NS_NewURI(getter_AddRefs(mManifestURI), uri);
   }

--- a/chrome/nsChromeRegistryChrome.h
+++ b/chrome/nsChromeRegistryChrome.h
@@ -36,21 +36,16 @@ class nsChromeRegistryChrome : public ns
   NS_IMETHOD IsLocaleRTL(const nsACString& package,
                          bool *aResult) override;
   NS_IMETHOD GetSelectedLocale(const nsACString& aPackage,
                                bool aAsBCP47,
                                nsACString& aLocale) override;
   NS_IMETHOD Observe(nsISupports *aSubject, const char *aTopic,
                      const char16_t *someData) override;
 
-#ifdef MOZ_XUL
-  NS_IMETHOD GetXULOverlays(nsIURI *aURI,
-                            nsISimpleEnumerator **_retval) override;
-#endif
-
   // If aChild is non-null then it is a new child to notify. If aChild is
   // null, then we have installed new chrome and we are resetting all of our
   // children's registered chrome.
   void SendRegisteredChrome(mozilla::dom::PContentParent* aChild);
 
  private:
   struct PackageEntry;
   static void ChromePackageFromPackageEntry(const nsACString& aPackageName,

--- a/chrome/nsChromeRegistryContent.cpp
+++ b/chrome/nsChromeRegistryContent.cpp
@@ -233,23 +233,16 @@ nsChromeRegistryContent::GetSelectedLoca
 
 NS_IMETHODIMP
 nsChromeRegistryContent::Observe(nsISupports* aSubject, const char* aTopic,
                                  const char16_t* aData)
 {
   CONTENT_NOT_IMPLEMENTED();
 }
 
-NS_IMETHODIMP
-nsChromeRegistryContent::GetXULOverlays(nsIURI *aChromeURL,
-                                        nsISimpleEnumerator **aResult)
-{
-  CONTENT_NOT_IMPLEMENTED();
-}
-
 void
 nsChromeRegistryContent::ManifestContent(ManifestProcessingContext& cx,
                                          int lineno, char *const * argv,
                                          int flags)
 {
   CONTENT_NOTREACHED();
 }
 

--- a/chrome/nsChromeRegistryContent.h
+++ b/chrome/nsChromeRegistryContent.h
@@ -30,18 +30,16 @@ class nsChromeRegistryContent : public n
   NS_IMETHOD CheckForOSAccessibility() override;
   NS_IMETHOD Observe(nsISupports* aSubject, const char* aTopic,
                      const char16_t* aData) override;
   NS_IMETHOD IsLocaleRTL(const nsACString& package,
                          bool *aResult) override;
   NS_IMETHOD GetSelectedLocale(const nsACString& aPackage,
                                bool aAsBCP47,
                                nsACString& aLocale) override;
-  NS_IMETHOD GetXULOverlays(nsIURI *aChromeURL,
-                            nsISimpleEnumerator **aResult) override;
 
   void RegisterPackage(const ChromePackage& aPackage);
   void RegisterOverride(const OverrideMapping& aOverride);
   void RegisterSubstitution(const SubstitutionMapping& aResource);
 
  private:
   struct PackageEntry
   {

--- a/dom/xul/XULDocument.cpp
+++ b/dom/xul/XULDocument.cpp
@@ -33,17 +33,16 @@
 #include "nsIContentViewer.h"
 #include "nsIStreamListener.h"
 #include "nsITimer.h"
 #include "nsDocShell.h"
 #include "nsGkAtoms.h"
 #include "nsXMLContentSink.h"
 #include "nsXULContentSink.h"
 #include "nsXULContentUtils.h"
-#include "nsIXULOverlayProvider.h"
 #include "nsIStringEnumerator.h"
 #include "nsDocElementCreatedNotificationRunner.h"
 #include "nsNetUtil.h"
 #include "nsParserCIID.h"
 #include "nsPIBoxObject.h"
 #include "mozilla/dom/BoxObject.h"
 #include "nsString.h"
 #include "nsPIDOMWindow.h"

--- a/dom/xul/moz.build
+++ b/dom/xul/moz.build
@@ -10,20 +10,16 @@ with Files("**"):
 if CONFIG['MOZ_BUILD_APP'] == 'browser':
     DEFINES['MOZ_BREAK_XUL_OVERLAYS'] = True
 
 MOCHITEST_MANIFESTS += ['test/mochitest.ini']
 
 MOCHITEST_CHROME_MANIFESTS += ['test/chrome.ini']
 
 if CONFIG['MOZ_XUL']:
-    XPIDL_SOURCES += [
-        'nsIXULOverlayProvider.idl',
-    ]
-
     EXPORTS += [
         'nsXULElement.h',
     ]
 
     UNIFIED_SOURCES += [
         'nsXULCommandDispatcher.cpp',
         'nsXULContentSink.cpp',
         'nsXULContentUtils.cpp',

deleted file mode 100644
--- a/dom/xul/nsIXULOverlayProvider.idl
+++ /dev/null
@@ -1,26 +0,0 @@
-/* -*- Mode: java; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "nsISupports.idl"
-
-interface nsISimpleEnumerator;
-interface nsIURI;
-
-/**
- * The chrome registry implements this interface to give overlays
- * to the gecko XUL engine.
- */
-
-[scriptable, uuid(1d5b5b94-dc47-4050-93b7-ac092e383cad)]
-interface nsIXULOverlayProvider : nsISupports
-{
-    /**
-     * Get the XUL overlays for a particular chrome URI.
-     *
-     * @param aURI  The URI being loaded
-     * @return      An enumerator of nsIURI for the overlays of this URI 
-     */
-    nsISimpleEnumerator /*nsIURI*/ getXULOverlays(in nsIURI aURI);
-};

--- a/xpcom/build/Services.py
+++ b/xpcom/build/Services.py
@@ -6,18 +6,16 @@ def service(name, iface, contractid):
     services.append((name, iface, contractid))
 
 service('ChromeRegistryService', 'nsIChromeRegistry',
         "@mozilla.org/chrome/chrome-registry;1")
 service('ToolkitChromeRegistryService', 'nsIToolkitChromeRegistry',
         "@mozilla.org/chrome/chrome-registry;1")
 service('XULChromeRegistryService', 'nsIXULChromeRegistry',
         "@mozilla.org/chrome/chrome-registry;1")
-service('XULOverlayProviderService', 'nsIXULOverlayProvider',
-        "@mozilla.org/chrome/chrome-registry;1")
 service('IOService', 'nsIIOService',
         "@mozilla.org/network/io-service;1")
 service('ObserverService', 'nsIObserverService',
         "@mozilla.org/observer-service;1")
 service('StringBundleService', 'nsIStringBundleService',
         "@mozilla.org/intl/stringbundle;1")
 service('XPConnect', 'nsIXPConnect',
         "@mozilla.org/js/xpc/XPConnect;1")
@@ -55,17 +53,16 @@ CPP_INCLUDES = """
 #include "nsNetCID.h"
 #include "nsObserverService.h"
 #include "nsXPCOMPrivate.h"
 #include "nsIIOService.h"
 #include "nsIDirectoryService.h"
 #include "nsIChromeRegistry.h"
 #include "nsIStringBundle.h"
 #include "nsIToolkitChromeRegistry.h"
-#include "nsIXULOverlayProvider.h"
 #include "IHistory.h"
 #include "nsIXPConnect.h"
 #include "nsIPermissionManager.h"
 #include "nsIServiceWorkerManager.h"
 #include "nsICacheStorageService.h"
 #include "nsIStreamTransportService.h"
 #include "nsISocketTransportService.h"
 #include "nsIURIClassifier.h"