Make quitter wait for idle startup finished

quitter-diff.txt

diff --git a/tools/quitter/parent.js b/tools/quitter/parent.js
--- a/tools/quitter/parent.js
+++ b/tools/quitter/parent.js
@@ -4,14 +4,24 @@
 
 const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm");
 
 this.quitter = class extends ExtensionAPI {
   getAPI(context) {
     return {
       quitter: {
         quit() {
+          let browserWindow = Services.wm.getMostRecentWindow("navigator:browser");
+          if (browserWindow && browserWindow.gBrowserInit &&
+              !browserWindow.gBrowserInit.idleTasksFinished) {
+            let obs = () => {
+              Services.obs.removeObserver(obs, "browser-idle-startup-tasks-finished");
+              this.quit();
+            };
+            Services.obs.addObserver(obs, "browser-idle-startup-tasks-finished");
+            return;
+          }
           Services.startup.quit(Ci.nsIAppStartup.eForceQuit);
         },
       },
     };
   }
 };