The @Url.Action() method is proccess on the server-side, so you cannot pass a client-side value to this function as a parameter. You can concat the client-side variables with the server-side url generated by this method, which is a string on the output. Try something like this:
var firstname = "abc";
var username = "abcd";
location.href = '@Url.Action("Display", "Customer")?uname=' + firstname + '&name=' + username;
The @Url.Action("Display", "Customer") is processed on the server-side and the rest of the string is processed on the client-side, concatenating the result of the server-side method with the client-side.