Firefox中的Ext.Ajax.request

I want to execute the following code in firefox

Ext.Ajax.request({
url: "http://localhost:1979/geoserver/j_spring_security_check?",
method: "POST",
scriptTag: true,
params: {
    username: username,
    password: password
},
success: function () {
    alert("ok");
}

});

For some reason this snippet of code is never executed in firefox or chrome (in ie works fine)

Thanks in advance