--- a/toolkit/components/maintenanceservice/workmonitor.cpp
+++ b/toolkit/components/maintenanceservice/workmonitor.cpp
@@ -234,27 +234,33 @@ StartUpdateProcess(int argc,
   // of error so that the normal update process can use it.
   if (selfHandlePostUpdate) {
     MoveFileExW(updaterINITemp, updaterINI, MOVEFILE_REPLACE_EXISTING);
 
     // Only run the PostUpdate if the update was successful
     if (updateWasSuccessful && argc > 2) {
       LPCWSTR installationDir = argv[2];
       LPCWSTR updateInfoDir = argv[1];
+      bool backgroundUpdate = (argc == 4 && !wcscmp(argv[3], L"-1"));
 
       // Launch the PostProcess with admin access in session 0.  This is
       // actually launching the post update process but it takes in the 
       // callback app path to figure out where to apply to.
       // The PostUpdate process with user only access will be done inside
       // the unelevated updater.exe after the update process is complete
       // from the service.  We don't know here which session to start
       // the user PostUpdate process from.
-      LOG(("Launching post update process as the service in session 0.\n"));
-      if (!LaunchWinPostProcess(installationDir, updateInfoDir, true, NULL)) {
-        LOG(("The post update process could not be launched.\n"));
+      // Note that we don't need to do this if we're just staging the
+      // update in the background, as the PostUpdate step runs when
+      // performing the replacing in that case.
+      if (!backgroundUpdate) {
+        LOG(("Launching post update process as the service in session 0.\n"));
+        if (!LaunchWinPostProcess(installationDir, updateInfoDir, true, NULL)) {
+          LOG(("The post update process could not be launched.\n"));
+        }
       }
     }
   }
 
   free(cmdLine);
   return updateWasSuccessful;
 }
 

--- a/toolkit/mozapps/update/updater/updater.cpp
+++ b/toolkit/mozapps/update/updater/updater.cpp
@@ -2423,17 +2423,20 @@ int NS_main(int argc, NS_tchar **argv)
         return 0;
       }
 
       // If we started the service command, and it finished, check the
       // update.status file to make sure it succeeded, and if it did
       // we need to manually start the PostUpdate process from the
       // current user's session of this unelevated updater.exe the
       // current process is running as.
-      if (useService) {
+      // Note that we don't need to do this if we're just staging the
+      // update in the background, as the PostUpdate step runs when
+      // performing the replacing in that case.
+      if (useService && !sBackgroundUpdate) {
         bool updateStatusSucceeded = false;
         if (IsUpdateStatusSucceeded(updateStatusSucceeded) && 
             updateStatusSucceeded) {
           if (!LaunchWinPostProcess(installDir, gSourcePath, false, NULL)) {
             fprintf(stderr, "The post update process which runs as the user"
                     " for service update could not be launched.");
           }
         }
@@ -2746,22 +2749,22 @@ int NS_main(int argc, NS_tchar **argv)
       // The service update will only be executed if it is already installed.
       // For first time installs of the service, the install will happen from
       // the PostUpdate process. We do the service update process here 
       // because it's possible we are updating with updater.exe without the 
       // service if the service failed to apply the update. We want to update
       // the service to a newer version in that case. If we are not running
       // through the service, then MOZ_USING_SERVICE will not exist.
       if (!usingService) {
-        if (!LaunchWinPostProcess(argv[callbackIndex], gSourcePath, false, NULL)) {
-          LOG(("NS_main: The post update process could not be launched.\n"));
-        }
-
         NS_tchar installDir[MAXPATHLEN];
         if (GetInstallationDir(installDir)) {
+          if (!LaunchWinPostProcess(installDir, gSourcePath, false, NULL)) {
+            LOG(("NS_main: The post update process could not be launched.\n"));
+          }
+
           StartServiceUpdate(installDir);
         }
       }
     }
     EXIT_WHEN_ELEVATED(elevatedLockFilePath, updateLockFileHandle, 0);
 #endif /* XP_WIN */
 #ifdef XP_MACOSX
     if (gSucceeded) {