--- a/js/xpconnect/loader/ScriptPreloader.cpp
+++ b/js/xpconnect/loader/ScriptPreloader.cpp
@@ -27,18 +27,19 @@
 #include "nsIFile.h"
 #include "nsIObserverService.h"
 #include "nsJSUtils.h"
 #include "nsProxyRelease.h"
 #include "nsThreadUtils.h"
 #include "nsXULAppAPI.h"
 #include "xpcpublic.h"
 
-#define DELAYED_STARTUP_TOPIC "browser-delayed-startup-finished"
+#define STARTUP_COMPLETE_TOPIC "browser-delayed-startup-finished"
 #define DOC_ELEM_INSERTED_TOPIC "document-element-inserted"
+#define CACHE_WRITE_TOPIC "browser-idle-startup-tasks-finished"
 #define CLEANUP_TOPIC "xpcom-shutdown"
 #define SHUTDOWN_TOPIC "quit-application-granted"
 #define CACHE_INVALIDATE_TOPIC "startupcache-invalidate"
 
 namespace mozilla {
 namespace {
 static LazyLogModule gLog("ScriptPreloader");
 
@@ -226,18 +227,19 @@ ScriptPreloader::ScriptPreloader()
         sProcessType = GetChildProcessType(dom::ContentChild::GetSingleton()->GetRemoteType());
     }
 
     nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
     MOZ_RELEASE_ASSERT(obs);
 
     if (XRE_IsParentProcess()) {
         // In the parent process, we want to freeze the script cache as soon
-        // as delayed startup for the first browser window has completed.
-        obs->AddObserver(this, DELAYED_STARTUP_TOPIC, false);
+        // as idle tasks for the first browser window have completed.
+        obs->AddObserver(this, STARTUP_COMPLETE_TOPIC, false);
+        obs->AddObserver(this, CACHE_WRITE_TOPIC, false);
     } else {
         // In the child process, we need to freeze the script cache before any
         // untrusted code has been executed. The insertion of the first DOM
         // document element may sometimes be earlier than is ideal, but at
         // least it should always be safe.
         obs->AddObserver(this, DOC_ELEM_INSERTED_TOPIC, false);
     }
     obs->AddObserver(this, SHUTDOWN_TOPIC, false);
@@ -333,22 +335,26 @@ ScriptPreloader::InvalidateCache()
                                     getter_AddRefs(mSaveThread), this);
     }
 }
 
 nsresult
 ScriptPreloader::Observe(nsISupports* subject, const char* topic, const char16_t* data)
 {
     nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
-    if (!strcmp(topic, DELAYED_STARTUP_TOPIC)) {
-        obs->RemoveObserver(this, DELAYED_STARTUP_TOPIC);
+    if (!strcmp(topic, STARTUP_COMPLETE_TOPIC)) {
+        obs->RemoveObserver(this, STARTUP_COMPLETE_TOPIC);
 
         MOZ_ASSERT(XRE_IsParentProcess());
 
         mStartupFinished = true;
+    } else if (!strcmp(topic, CACHE_WRITE_TOPIC)) {
+        obs->RemoveObserver(this, CACHE_WRITE_TOPIC);
+        MOZ_ASSERT(mStartupFinished);
+        MOZ_ASSERT(XRE_IsParentProcess());
 
         if (mChildCache) {
             Unused << NS_NewNamedThread("SaveScripts",
                                         getter_AddRefs(mSaveThread), this);
         }
     } else if (!strcmp(topic, DOC_ELEM_INSERTED_TOPIC)) {
         obs->RemoveObserver(this, DOC_ELEM_INSERTED_TOPIC);