--- a/js/ipc/JavaScriptParent.cpp
+++ b/js/ipc/JavaScriptParent.cpp
@@ -53,89 +53,39 @@ ForbidUnsafeBrowserCPOWs()
     static bool cached = false;
     if (!cached) {
         cached = true;
         Preferences::AddBoolVarCache(&result, "dom.ipc.cpows.forbid-unsafe-from-browser", false);
     }
     return result;
 }
 
-// Should we allow CPOWs in aAddonId, even though it's marked as multiprocess
-// compatible? This is controlled by two prefs:
-//   If dom.ipc.cpows.forbid-cpows-in-compat-addons is false, then we allow the CPOW.
-//   If dom.ipc.cpows.forbid-cpows-in-compat-addons is true:
-//     We check if aAddonId is listed in dom.ipc.cpows.allow-cpows-in-compat-addons
-//     (which should be a comma-separated string). If it's present there, we allow
-//     the CPOW. Otherwise we forbid the CPOW.
-static bool
-ForbidCPOWsInCompatibleAddon(const nsACString& aAddonId)
-{
-    bool forbid = Preferences::GetBool("dom.ipc.cpows.forbid-cpows-in-compat-addons", false);
-    if (!forbid) {
-        return false;
-    }
-
-    nsAutoCString allow;
-    allow.Assign(',');
-    nsAutoCString pref;
-    Preferences::GetCString("dom.ipc.cpows.allow-cpows-in-compat-addons", pref);
-    allow.Append(pref);
-    allow.Append(',');
-
-    nsCString searchString(",");
-    searchString.Append(aAddonId);
-    searchString.Append(',');
-    return allow.Find(searchString) == kNotFound;
-}
-
 bool
 JavaScriptParent::allowMessage(JSContext* cx)
 {
     // If we're running browser code, then we allow all safe CPOWs and forbid
     // unsafe CPOWs based on a pref (which defaults to forbidden). We also allow
     // CPOWs unconditionally in selected globals (based on
     // Cu.permitCPOWsInScope).
-    //
-    // If we're running add-on code, then we check if the add-on is multiprocess
-    // compatible (which eventually translates to a given setting of allowCPOWs
-    // on the scopw). If it's not compatible, then we allow the CPOW but
-    // warn. If it is marked as compatible, then we check the
-    // ForbidCPOWsInCompatibleAddon; see the comment there.
 
     MessageChannel* channel = GetIPCChannel();
     bool isSafe = channel->IsInTransaction();
 
     bool warn = !isSafe;
     nsIGlobalObject* global = dom::GetIncumbentGlobal();
     JS::Rooted<JSObject*> jsGlobal(cx, global ? global->GetGlobalJSObject() : nullptr);
     if (jsGlobal) {
         JSAutoCompartment ac(cx, jsGlobal);
-        JSAddonId* addonId = JS::AddonIdOfObject(jsGlobal);
 
         if (!xpc::CompartmentPrivate::Get(jsGlobal)->allowCPOWs) {
-            if (!addonId && ForbidUnsafeBrowserCPOWs() && !isSafe) {
+            if (ForbidUnsafeBrowserCPOWs() && !isSafe) {
                 Telemetry::Accumulate(Telemetry::BROWSER_SHIM_USAGE_BLOCKED, 1);
                 JS_ReportErrorASCII(cx, "unsafe CPOW usage forbidden");
                 return false;
             }
-
-            if (addonId) {
-                JSFlatString* flat = JS_ASSERT_STRING_IS_FLAT(JS::StringOfAddonId(addonId));
-                nsString addonIdString;
-                AssignJSFlatString(addonIdString, flat);
-                NS_ConvertUTF16toUTF8 addonIdCString(addonIdString);
-                Telemetry::Accumulate(Telemetry::ADDON_FORBIDDEN_CPOW_USAGE, addonIdCString);
-
-                if (ForbidCPOWsInCompatibleAddon(addonIdCString)) {
-                    JS_ReportErrorASCII(cx, "CPOW usage forbidden in this add-on");
-                    return false;
-                }
-
-                warn = true;
-            }
         }
     }
 
     if (!warn)
         return true;
 
     static bool disableUnsafeCPOWWarnings = PR_GetEnv("DISABLE_UNSAFE_CPOW_WARNINGS");
     if (!disableUnsafeCPOWWarnings) {

--- a/js/xpconnect/idl/xpccomponents.idl
+++ b/js/xpconnect/idl/xpccomponents.idl
@@ -685,19 +685,16 @@ interface nsIXPCComponents_Utils : nsISu
      * Unwraps cross-compartment wrappers first.
      *
      * The string formats and values may change at any time. Do not depend on
      * this from addon code.
      */
     [implicit_jscontext]
     ACString getCompartmentLocation(in jsval obj);
 
-    [implicit_jscontext]
-    void allowCPOWsInAddon(in ACString addonId, in bool allow);
-
     /*
      * Return a fractional number of milliseconds from process
      * startup, measured with a monotonic clock.
      */
     double now();
 
     /*
      * Reads the given file and returns its contents. If called during early

--- a/js/xpconnect/src/XPCComponents.cpp
+++ b/js/xpconnect/src/XPCComponents.cpp
@@ -3101,30 +3101,16 @@ nsXPCComponents_Utils::GetCompartmentLoc
     obj = js::CheckedUnwrap(obj);
     MOZ_ASSERT(obj);
 
     result = xpc::CompartmentPrivate::Get(obj)->GetLocation();
     return NS_OK;
 }
 
 NS_IMETHODIMP
-nsXPCComponents_Utils::AllowCPOWsInAddon(const nsACString& addonIdStr,
-                                         bool allow,
-                                         JSContext* cx)
-{
-    JSAddonId* addonId = xpc::NewAddonId(cx, addonIdStr);
-    if (!addonId)
-        return NS_ERROR_FAILURE;
-    if (!XPCWrappedNativeScope::AllowCPOWsInAddon(cx, addonId, allow))
-        return NS_ERROR_FAILURE;
-
-    return NS_OK;
-}
-
-NS_IMETHODIMP
 nsXPCComponents_Utils::ReadUTF8File(nsIFile* aFile, nsACString& aResult)
 {
     NS_ENSURE_TRUE(aFile, NS_ERROR_INVALID_ARG);
 
     MOZ_TRY_VAR(aResult, URLPreloader::ReadFile(aFile));
     return NS_OK;
 }
 

--- a/js/xpconnect/src/XPCWrappedNativeScope.cpp
+++ b/js/xpconnect/src/XPCWrappedNativeScope.cpp
@@ -21,36 +21,16 @@
 using namespace mozilla;
 using namespace xpc;
 using namespace JS;
 
 /***************************************************************************/
 
 XPCWrappedNativeScope* XPCWrappedNativeScope::gScopes = nullptr;
 XPCWrappedNativeScope* XPCWrappedNativeScope::gDyingScopes = nullptr;
-bool XPCWrappedNativeScope::gShutdownObserverInitialized = false;
-XPCWrappedNativeScope::AddonSet* XPCWrappedNativeScope::gAllowCPOWAddonSet = nullptr;
-
-NS_IMPL_ISUPPORTS(XPCWrappedNativeScope::ClearInterpositionsObserver, nsIObserver)
-
-NS_IMETHODIMP
-XPCWrappedNativeScope::ClearInterpositionsObserver::Observe(nsISupports* subject,
-                                                            const char* topic,
-                                                            const char16_t* data)
-{
-    MOZ_ASSERT(strcmp(topic, NS_XPCOM_SHUTDOWN_OBSERVER_ID) == 0);
-
-    if (gAllowCPOWAddonSet) {
-        delete gAllowCPOWAddonSet;
-        gAllowCPOWAddonSet = nullptr;
-    }
-
-    nsContentUtils::UnregisterShutdownObserver(this);
-    return NS_OK;
-}
 
 static bool
 RemoteXULForbidsXBLScope(nsIPrincipal* aPrincipal, HandleObject aGlobal)
 {
   MOZ_ASSERT(aPrincipal);
 
   // Certain singleton sandoxes are created very early in startup - too early
   // to call into AllowXULXBLForPrincipal. We never create XBL scopes for
@@ -121,23 +101,16 @@ XPCWrappedNativeScope::XPCWrappedNativeS
     mUseContentXBLScope = mAllowContentXBLScope;
     if (mUseContentXBLScope) {
         const js::Class* clasp = js::GetObjectClass(mGlobalJSObject);
         mUseContentXBLScope = !strcmp(clasp->name, "Window");
     }
     if (mUseContentXBLScope) {
         mUseContentXBLScope = principal && !nsContentUtils::IsSystemPrincipal(principal);
     }
-
-    if (JSAddonId* addonId = JS::AddonIdOfObject(aGlobal)) {
-        // We forbid CPOWs unless they're specifically allowed.
-        priv->allowCPOWs = gAllowCPOWAddonSet ? gAllowCPOWAddonSet->has(addonId) : false;
-        MOZ_ASSERT(!mozJSComponentLoader::Get()->IsLoaderGlobal(aGlobal),
-                   "Don't load addons into the shared JSM global");
-    }
 }
 
 // static
 bool
 XPCWrappedNativeScope::IsDyingScope(XPCWrappedNativeScope* scope)
 {
     for (XPCWrappedNativeScope* cur = gDyingScopes; cur; cur = cur->mNext) {
         if (scope == cur)
@@ -728,40 +701,16 @@ XPCWrappedNativeScope::SetExpandoChain(J
     MOZ_ASSERT(ObjectScope(target) == this);
     MOZ_ASSERT(js::IsObjectInContextCompartment(target, cx));
     MOZ_ASSERT_IF(chain, ObjectScope(chain) == this);
     if (!mXrayExpandos.initialized() && !mXrayExpandos.init(cx))
         return false;
     return mXrayExpandos.put(cx, target, chain);
 }
 
-/* static */ bool
-XPCWrappedNativeScope::AllowCPOWsInAddon(JSContext* cx,
-                                         JSAddonId* addonId,
-                                         bool allow)
-{
-    if (!gAllowCPOWAddonSet) {
-        gAllowCPOWAddonSet = new AddonSet();
-        bool ok = gAllowCPOWAddonSet->init();
-        NS_ENSURE_TRUE(ok, false);
-
-        if (!gShutdownObserverInitialized) {
-            gShutdownObserverInitialized = true;
-            nsContentUtils::RegisterShutdownObserver(new ClearInterpositionsObserver());
-        }
-    }
-    if (allow) {
-        bool ok = gAllowCPOWAddonSet->put(addonId);
-        NS_ENSURE_TRUE(ok, false);
-    } else {
-        gAllowCPOWAddonSet->remove(addonId);
-    }
-    return true;
-}
-
 
 /***************************************************************************/
 
 // static
 void
 XPCWrappedNativeScope::DebugDumpAllScopes(int16_t depth)
 {
 #ifdef DEBUG

--- a/js/xpconnect/src/nsXPConnect.cpp
+++ b/js/xpconnect/src/nsXPConnect.cpp
@@ -1186,31 +1186,16 @@ JSAddonId*
 NewAddonId(JSContext* cx, const nsACString& id)
 {
     JS::RootedString str(cx, JS_NewStringCopyN(cx, id.BeginReading(), id.Length()));
     if (!str)
         return nullptr;
     return JS::NewAddonId(cx, str);
 }
 
-bool
-AllowCPOWsInAddon(const nsACString& addonIdStr, bool allow)
-{
-    JSAddonId* addonId;
-    // We enter the junk scope just to allocate a string, which actually will go
-    // in the system zone.
-    AutoJSAPI jsapi;
-    if (!jsapi.Init(xpc::PrivilegedJunkScope()))
-        return false;
-    addonId = NewAddonId(jsapi.cx(), addonIdStr);
-    if (!addonId)
-        return false;
-    return XPCWrappedNativeScope::AllowCPOWsInAddon(jsapi.cx(), addonId, allow);
-}
-
 } // namespace xpc
 
 namespace mozilla {
 namespace dom {
 
 bool
 IsChromeOrXBL(JSContext* cx, JSObject* /* unused */)
 {

--- a/js/xpconnect/src/xpcprivate.h
+++ b/js/xpconnect/src/xpcprivate.h
@@ -969,20 +969,16 @@ public:
     AddSizeOfAllScopesIncludingThis(ScopeSizeInfo* scopeSizeInfo);
 
     void
     AddSizeOfIncludingThis(ScopeSizeInfo* scopeSizeInfo);
 
     static bool
     IsDyingScope(XPCWrappedNativeScope* scope);
 
-    typedef js::HashSet<JSAddonId*,
-                        js::PointerHasher<JSAddonId*>,
-                        js::SystemAllocPolicy> AddonSet;
-
     // Gets the appropriate scope object for XBL in this scope. The context
     // must be same-compartment with the global upon entering, and the scope
     // object is wrapped into the compartment of the global.
     JSObject* EnsureContentXBLScope(JSContext* cx);
 
     JSObject* EnsureAddonScope(JSContext* cx, JSAddonId* addonId);
 
     XPCWrappedNativeScope(JSContext* cx, JS::HandleObject aGlobal);
@@ -992,39 +988,25 @@ public:
     JSCompartment* Compartment() const { return js::GetObjectCompartment(mGlobalJSObject); }
 
     bool IsContentXBLScope() { return xpc::IsContentXBLCompartment(Compartment()); }
     bool AllowContentXBLScope();
     bool UseContentXBLScope() { return mUseContentXBLScope; }
     void ClearContentXBLScope() { mContentXBLScope = nullptr; }
 
     bool IsAddonScope() { return xpc::IsAddonCompartment(Compartment()); }
-
-    static bool AllowCPOWsInAddon(JSContext* cx, JSAddonId* addonId, bool allow);
-
 protected:
     virtual ~XPCWrappedNativeScope();
 
     XPCWrappedNativeScope() = delete;
 
 private:
-    class ClearInterpositionsObserver final : public nsIObserver {
-        ~ClearInterpositionsObserver() {}
-
-      public:
-        NS_DECL_ISUPPORTS
-        NS_DECL_NSIOBSERVER
-    };
-
     static XPCWrappedNativeScope* gScopes;
     static XPCWrappedNativeScope* gDyingScopes;
 
-    static bool                      gShutdownObserverInitialized;
-    static AddonSet*                 gAllowCPOWAddonSet;
-
     Native2WrappedNativeMap*         mWrappedNativeMap;
     ClassInfo2WrappedNativeProtoMap* mWrappedNativeProtoMap;
     RefPtr<nsXPCComponentsBase>    mComponents;
     XPCWrappedNativeScope*           mNext;
     // The JS global object for this scope.  If non-null, this will be the
     // default parent for the XPCWrappedNatives that have us as the scope,
     // unless a PreCreate hook overrides it.  Note that this _may_ be null (see
     // constructor).

--- a/js/xpconnect/src/xpcpublic.h
+++ b/js/xpconnect/src/xpcpublic.h
@@ -544,22 +544,16 @@ SimulateActivityCallback(bool aActive);
 // racey.
 bool
 ShouldDiscardSystemSource();
 
 bool
 SharedMemoryEnabled();
 
 bool
-SetAddonInterposition(const nsACString& addonId, nsIAddonInterposition* interposition);
-
-bool
-AllowCPOWsInAddon(const nsACString& addonId, bool allow);
-
-bool
 ExtraWarningsForSystemJS();
 
 class ErrorBase {
   public:
     nsString mErrorMsg;
     nsString mFileName;
     uint32_t mLineNumber;
     uint32_t mColumn;