I have a modified shopping cart quantity update code in my tpl_shopping_cart_default.php, it looks like this:

<?php echo zen_draw_form('cart_quantity', zen_href_link(FILENAME_SHOPPING_CART, 'action=update_product')); ?>
<div id="cartInstructionsDisplay" class="content"><?php echo TEXT_INFORMATION; ?></div>

<?php if (!empty($totalsDisplay)) { ?>
<div class="cartTotalsDisplay important"><?php echo $totalsDisplay; ?></div>

It updates when you click "Update", but it also updates the quantity automatically if you click "Checkout" forgetting to click "Update" first, and then goes to the login page.

This works perfectly fine in Internet Explorer, but when I click "Update" in Firefox it goes to the login page instead of updating the quantity.
Anybody who have any idea what is wrong with the code??
Website is www.usconverters.com.