Bug 462077 - Lazily create Extensions object (FUEL)

This lazily creates the Extensions object off of Application so we do not take
the hit of initializing the object during startup if we do not need it.
r=gavin

toolkit/components/exthelper/extApplication.js

@@ -616,7 +616,6 @@ extApplication.prototype = {
   // for nsIObserver
   observe: function app_observe(aSubject, aTopic, aData) {
     if (aTopic == "app-startup") {
-      this._extensions = new Extensions();
       this.events.dispatch("load", "application");
     }
     else if (aTopic == "final-ui-startup") {
@@ -675,6 +674,9 @@ extApplication.prototype = {
   },
 
   get extensions() {
+    if (this._extensions == null)
+      this._extensions = new Extensions();
+
     return this._extensions;
   },