let Tabs = {
     
        _browser: function () {            
            var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"] 
                 .getService(Components.interfaces.nsIWindowMediator);        
            return wm.getMostRecentWindow("navigator:browser").gBrowser;
        }(),
     
        go: function (tabIndex) {
            this._browser.selectedTab = this._browser.tabContainer.childNodes[tabIndex - 1];    
        }     
    };

Tabs.go(2);