diff --git a/js/xpconnect/loader/ScriptPreloader.cpp b/js/xpconnect/loader/ScriptPreloader.cpp
index b22ed3161ce6..1a0879d8ff63 100644
--- a/js/xpconnect/loader/ScriptPreloader.cpp
+++ b/js/xpconnect/loader/ScriptPreloader.cpp
@@ -6,6 +6,7 @@
 
 #include "ScriptPreloader-inl.h"
 #include "mozilla/ScriptPreloader.h"
+#include "mozJSComponentLoader.h"
 #include "mozilla/loader/ScriptCacheActors.h"
 
 #include "mozilla/URLPreloader.h"
@@ -907,6 +908,12 @@ ScriptPreloader::DoFinishOffThreadDecode()
     MaybeFinishOffThreadDecode();
 }
 
+JSObject*
+ScriptPreloader::CompilationScope(JSContext* cx)
+{
+    return mozJSComponentLoader::Get()->CompilationScope(cx);
+}
+
 void
 ScriptPreloader::MaybeFinishOffThreadDecode()
 {
@@ -922,10 +929,10 @@ ScriptPreloader::MaybeFinishOffThreadDecode()
         DecodeNextBatch(OFF_THREAD_CHUNK_SIZE);
     });
 
-    AutoJSAPI jsapi;
-    MOZ_RELEASE_ASSERT(jsapi.Init(xpc::CompilationScope()));
-
+    AutoSafeJSAPI jsapi;
     JSContext* cx = jsapi.cx();
+
+    JSAutoCompartment ac(cx, CompilationScope(cx));
     JS::Rooted<JS::ScriptVector> jsScripts(cx, JS::ScriptVector(cx));
 
     // If this fails, we still need to mark the scripts as finished. Any that
@@ -993,9 +1000,9 @@ ScriptPreloader::DecodeNextBatch(size_t chunkSize)
         return;
     }
 
-    AutoJSAPI jsapi;
-    MOZ_RELEASE_ASSERT(jsapi.Init(xpc::CompilationScope()));
+    AutoSafeJSAPI jsapi;
     JSContext* cx = jsapi.cx();
+    JSAutoCompartment ac(cx, CompilationScope(cx));
 
     JS::CompileOptions options(cx, JSVERSION_DEFAULT);
     options.setNoScriptRval(true)
diff --git a/js/xpconnect/loader/ScriptPreloader.h b/js/xpconnect/loader/ScriptPreloader.h
index ee89632e5c7b..4c83b0736847 100644
--- a/js/xpconnect/loader/ScriptPreloader.h
+++ b/js/xpconnect/loader/ScriptPreloader.h
@@ -390,6 +390,11 @@ private:
     void MaybeFinishOffThreadDecode();
     void DoFinishOffThreadDecode();
 
+    // Returns the global scope object for off-thread compilation. When global
+    // sharing is enabled in the component loader, this should be the shared
+    // module global. Otherwise, it should be the XPConnect compilation scope.
+    JSObject* CompilationScope(JSContext* cx);
+
     size_t ShallowHeapSizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf)
     {
         return (mallocSizeOf(this) + mScripts.ShallowSizeOfExcludingThis(mallocSizeOf) +
diff --git a/js/xpconnect/loader/mozJSComponentLoader.cpp b/js/xpconnect/loader/mozJSComponentLoader.cpp
index b8424742f4b7..c5b441025661 100644
--- a/js/xpconnect/loader/mozJSComponentLoader.cpp
+++ b/js/xpconnect/loader/mozJSComponentLoader.cpp
@@ -301,6 +301,8 @@ NS_IMPL_ISUPPORTS(mozJSComponentLoader,
 nsresult
 mozJSComponentLoader::ReallyInit()
 {
+    MOZ_ASSERT(!mInitialized);
+
     mShareLoaderGlobal = Preferences::GetBool("jsloader.shareGlobal");
 
     nsresult rv;
@@ -468,7 +470,7 @@ mozJSComponentLoader::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf)
 
 void
 mozJSComponentLoader::CreateLoaderGlobal(JSContext* aCx,
-                                         nsACString& aLocation,
+                                         const nsACString& aLocation,
                                          JSAddonId* aAddonID,
                                          MutableHandleObject aGlobal)
 {
@@ -550,6 +552,31 @@ mozJSComponentLoader::ReuseGlobal(bool aIsAddon, nsIURI* aURI)
     return true;
 }
 
+JSObject*
+mozJSComponentLoader::GetSharedGlobal(JSContext* aCx)
+{
+    if (!mLoaderGlobal) {
+        MOZ_ASSERT(mShareLoaderGlobal);
+
+        JS::RootedObject globalObj(aCx);
+        CreateLoaderGlobal(aCx, NS_LITERAL_CSTRING("shared JSM global"),
+                           nullptr, &globalObj);
+
+        // If we fail to create a module global this early, we're not going to
+        // get very far, so just bail out now.
+        MOZ_RELEASE_ASSERT(globalObj);
+        mLoaderGlobal = globalObj;
+
+        // AutoEntryScript required to invoke debugger hook, which is a
+        // Gecko-specific concept at present.
+        dom::AutoEntryScript aes(globalObj,
+                                 "component loader report global");
+        JS_FireOnNewGlobalObject(aes.cx(), globalObj);
+    }
+
+    return mLoaderGlobal;
+}
+
 JSObject*
 mozJSComponentLoader::PrepareObjectForLocation(JSContext* aCx,
                                                nsIFile* aComponentFile,
@@ -567,25 +594,10 @@ mozJSComponentLoader::PrepareObjectForLocation(JSContext* aCx,
 
     bool createdNewGlobal = false;
     RootedObject globalObj(aCx);
-    if (reuseGlobal)
-        globalObj = mLoaderGlobal;
-
-    if (!globalObj) {
-        nsAutoCString globalLocation;
-        if (reuseGlobal) {
-            globalLocation.AssignLiteral("shared JSM global");
-        } else {
-            globalLocation.Assign(nativePath);
-        }
-
-        CreateLoaderGlobal(aCx, globalLocation,
-                           addonId, &globalObj);
-        NS_ENSURE_TRUE(globalObj, nullptr);
-
-        if (reuseGlobal) {
-            mLoaderGlobal = globalObj;
-        }
-
+    if (reuseGlobal) {
+        globalObj = GetSharedGlobal(aCx);
+    } else if (!globalObj) {
+        CreateLoaderGlobal(aCx, nativePath, addonId, &globalObj);
         createdNewGlobal = true;
     }
 
diff --git a/js/xpconnect/loader/mozJSComponentLoader.h b/js/xpconnect/loader/mozJSComponentLoader.h
index 7dbbdc758c7c..6ef4a47925b3 100644
--- a/js/xpconnect/loader/mozJSComponentLoader.h
+++ b/js/xpconnect/loader/mozJSComponentLoader.h
@@ -20,12 +20,18 @@
 #include "jsapi.h"
 
 #include "xpcIJSGetFactory.h"
+#include "xpcpublic.h"
 
 class nsIFile;
 class nsIPrincipal;
 class nsIXPConnectJSObjectHolder;
 class ComponentLoaderInfo;
 
+namespace mozilla {
+    class ScriptPreloader;
+} // namespace mozilla
+
+
 /* 6bd13476-1dd2-11b2-bbef-f0ccb5fa64b6 (thanks, mozbot) */
 
 #define MOZJSCOMPONENTLOADER_CID                                              \
@@ -71,12 +77,28 @@ class mozJSComponentLoader : public mozilla::ModuleLoader,
     void UnloadModules();
 
     void CreateLoaderGlobal(JSContext* aCx,
-                            nsACString& aLocation,
+                            const nsACString& aLocation,
                             JSAddonId* aAddonID,
                             JS::MutableHandleObject aGlobal);
 
     bool ReuseGlobal(bool aIsAddon, nsIURI* aComponent);
 
+    friend class mozilla::ScriptPreloader;
+
+    JSObject* CompilationScope(JSContext* aCx)
+    {
+        if (mLoaderGlobal)
+            return mLoaderGlobal;
+        if ((mInitialized || NS_SUCCEEDED(ReallyInit())) &&
+            mShareLoaderGlobal)
+        {
+            return GetSharedGlobal(aCx);
+        }
+        return xpc::CompilationScope();
+    }
+
+    JSObject* GetSharedGlobal(JSContext* aCx);
+
     JSObject* PrepareObjectForLocation(JSContext* aCx,
                                        nsIFile* aComponentFile,
                                        nsIURI* aComponent,