Setting Context.Response.StatusCode does not seem to work

Question

I have an HttpHandler with the following code:

using System;
using System.Web;
using Company.Cms;
using Company.Web.Handlers.Console;


namespace Company.Web.Handlers
{
    /// <summary>
    /// Summary description for AdminHandler
    /// </summary>
    public class AdminHandler : IHttpHandler
    {
        public bool IsReusable
        {
            get
            {
                return false;
            }
        }

        public void ProcessRequest(HttpContext context)
        {
            HttpRequest request = context.Request;
            HttpResponse response = context.Response;
            string action = request.QueryString["action"];

            if (!HttpContext.Current.User.CanAdminister())
            {
                response.StatusCode = 403;
                response.Status = "403 Access Denied";
                response.End();
                return;
            }

            if (String.IsNullOrEmpty(action))
            {
                response.StatusCode = 404;
                response.Status = "404 Not Found";
                response.End();
                return;
            }

            IHttpHandler handler = null;
            switch (action)
            {
                case "menu":
                    handler = new MenuHandler();
                    break;
                case "tree":
                    handler = new TreeHandler();
                    break;
                case "grid":
                    handler = new GridHandler();
                    break;
                case "list":
                    handler = new ListHandler();
                    break;
                case "drop":
                    handler = new DropHandler();
                    break;
                case "edit":
                    handler = new EditHandler();
                    break;
                case "new":
                    handler = new InsertHandler();
                    break;
            }
            if (handler == null)
            {
                response.StatusCode = 404;
                response.Status = "404 Not Found";
                response.End();
            }
            else
            {
                handler.ProcessRequest(context);
            }
        }
    }
}

Unfortunately when I intentionally specify an invalid action, the browser just displays a blank page. Non of the browser error messages are displayed both in Firefox and IE.

What could I be doing wrong?

EDIT - IE shows the error message, but Firefox does not.