ReCaptcha ne fonctionne pas sur Firefox mais sur IE

<div class="contactform_line"><form action="envoi.php" method="post" enctype="application/x-www-form-urlencoded" name="formulaire">
<strong>Send us a message</strong></div>
<div class="contactform_line">Your name:<input type="text" name="nom" size="45" maxlength="100"></div>
<div class="contactform_line">Your email adress:<input type="text" name="mail" size="45" maxlength="100"></div>
<div class="contactform_line">Object:<input type="text" name="objet" size="45" maxlength="100"></div>
<div class="contactform_line_message">Message:<textarea id="textarea_size" name="message" cols="50" rows="10"></textarea></div>
<div class="contactform_line_message"><?php
require_once('recaptchalib.php');
$publickey = "...";
echo recaptcha_get_html($publickey);
?></div>
<div class="contactform_line">
<input type="submit" name="Submit" value="Send"><input type="reset" name="Submit" value="Reset the form"></div>

<?php
require_once('recaptchalib.php');
$privatekey = "...";
$resp = recaptcha_check_answer ($privatekey,
$_SERVER["REMOTE_ADDR"],
$_POST["recaptcha_challenge_field"],
$_POST["recaptcha_response_field"]);

if (!$resp->is_valid) { // Test si le captcha a bien été rempli
// Si le captcha n'est pas valide
echo 'Oups, le captcha antispam n\'est pas valide. Veuillez recommencer';
}
else{
// Si le captcha est valide
$nom=$HTTP_POST_VARS['nom'];
$mail=$HTTP_POST_VARS['mail'];
$objet=$HTTP_POST_VARS['objet'];
$message=$HTTP_POST_VARS['message'];

/////voici la version Mine
$headers = "MIME-Version: 1.0\r\n";

//////ici on détermine le mail en format text
$headers .= "Content-type: text/plain; charset=iso-8859-1\r\n";

////ici on détermine l'expediteur et l'adresse de réponse
$headers .= "From: $nom <$mail>\r\nReply-to : $nom <$mail>\nX-Mailer:PHP";

$subject="$objet";
$destinataire="contact@contact.com";
$body="$message";
if (mail($destinataire,$subject,$body,$headers)) {
echo "Thank you, your message has been sent<br>";
} else {
echo "Sorry, your message has not be sent to a technical issue";
}
?>
<?php echo "You will be automatically redirected to the index page in 3 secondes.","\n"; ?> <p></p>
<?php echo '<meta http-equiv="refresh" content="3; url=http://www.contact.com">';
}
?>