MVC's And Firefox's Cache Work?

I use the following code to write cache header on *.png requests:

response.Buffer = false;
response.BufferOutput = false;
// Emit content type and encoding based on the file extension and
// whether the response is compressed
response.ContentType = MimeMapping.GetMimeMapping(physicalFilePath);
if (mode != ResponseCompressionType.None)
response.AppendHeader("Content-Encoding", mode.ToString().ToLower());
response.AppendHeader("Content-Length", count.ToString());
// Emit proper cache headers that will cache the response in browser's
// cache for the default cache duration
response.Cache.SetCacheability(HttpCacheability.Public);
response.Cache.AppendCacheExtension("must-revalidate, proxy-revalidate");
response.Cache.SetMaxAge(DEFAULT_CACHE_DURATION);
response.Cache.SetExpires(DateTime.Now.Add(DEFAULT_CACHE_DURATION));
response.Cache.SetLastModified(lastModified);

But every time I refresh the page which contains the PNG URL, it will post to web server again. It seems that the cache header not work, and worse, it make the browser cache not work too. I am using asp.net mvc.