firefox shutdown hook example

firefox_shutdown_hook.js

var ShutdownHook = {
  
  prefs: PrefsService, // wrapper around XPCOM preferences API

  register: function() {
    if (this.prefs.shutdownRegistered) {
      return;
    }
  
    var observerService = 
         Components.classes["@mozilla.org/observer-service;1"]
           .getService (Components.interfaces.nsIObserverService);
  
    observerService.addObserver(this, "quit-application-granted", false);
  
    this.prefs.shutdownRegistered = true;
    this.prefs.set("shutdownRegistered", true);
  },

  observe: function(subject, topic, data) {
    if (topic != "quit-application-granted" ) {
      return;
    }
    
    // do something at shutdown here...
    // ... code ...
    // ... code ...
    
    // reset var so it can be hooked again the next time 
    // firefox is launched
    this.prefs.set("shutdownRegistered", false);
  },

  unregister: function() {
    try {
      if (!this.prefs.shutdownRegistered) {
        return;
      }
      var observerService = 
           Components.classes["@mozilla.org/observer-service;1"]
             .getService (Components.interfaces.nsIObserverService);
              
      observerService.removeObserver(this, "quit-application-granted");
    
      this.prefs.shutdownRegistered = false;
      this.prefs.set("shutdownRegistered", false);
    } catch (err) {
    }
  },

};

ShutdownHook.register();