var data = { FirstName: 'Alex' };

            $.ajax({
                url: "/Account/AddFirstProducts",

                data: JSON.stringify(data),
                type: 'POST',
                dataType: 'json',
                contentType: 'application/json; charset=utf-8',
                success: function (data) { alert(data); },
                failure: function (errMsg) {
                    alert(errMsg);
                }
            });

  public class FromJsonAttribute : CustomModelBinderAttribute
    {
      private readonly static JavaScriptSerializer serializer = new JavaScriptSerializer();
      public override IModelBinder GetBinder()
      {
         return new JsonModelBinder();
      }

      private class JsonModelBinder : IModelBinder
      {
         public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
         {
           var stringified = controllerContext.HttpContext.Request[bindingContext.ModelName];
          
[h2]// stringified ВСЕГДА NULL[/h2]
 		if (string.IsNullOrEmpty(stringified))
                return null;
           return serializer.Deserialize(stringified, bindingContext.ModelType);
          }
     }
 }

public ActionResult AddFirstName([FromJson] string FirstName)
{
   // FirstName равен NULL
   Debug.WriteLine(FirstName);
   return Json(new { Success = true, Message = "ok" });     
}

  $(function() {$.ajax({
                url: "/Account/AddFirstProducts",

                data: JSON.stringify(data),
                type: 'POST',
                dataType: 'JSON',
                processData: false,
                contentType: 'application/json; charset=utf-8',
                success: function (data) { alert(data); },
                failure: function (errMsg) {
                    alert(errMsg);
                }
            });
            });