Image verification code not working firefox

[vbbit]

I found this tutorial, but I don't know why it does not work with firefox. It works fine with IE, can anyone help please?

<?php
// initialize session
session_start();
// send header so telling the connectiont that we are about to send an image
header("Content-Type: image/png");
header('Cache-Control: no-cache');

// generate 5 digits random number
$rand = rand(10000, 99999);

// create the hash for the random numbers and put them in the session
$_SESSION['img_random_value'] = md5($rand);

// create the image
$image = imagecreate(60, 30);

// set the background color to white - this has to come first
$bgColor = imagecolorallocate($image, 255, 255, 255);

// set the text color to black
$txtColor = imagecolorallocate($image, 0, 0, 0);

// write the image
//imagefill($image, 0, 0, $bgColor);
imagestring($image, 5, 5, 8, $rand, $txtColor);

// output it to browser
imagepng($image, "verify.png");

// destroy the image to free up the server memory
imagedestroy($image);


?>