Firefox cookie not transmitted (socket.io authorization)

I have this code where I check a cookie send from the client

io.set('authorization', function (data, accept) {

    if (data.headers.cookie) {
       //... do something with the cookie...

    } else 
    {
       return accept('No cookie transmitted.', false);
    }

});

When I use chrome or iexplorer data.headers.cookie is defined, but if I use firefox ( 17.0.1) the cookie isn´t transmitted and data.headers.cookie is undefined.

Anybody knows what is the problem?