Text not centrally aligned in IE and FF

Works fine in Chrome/Safari, but in FF and IE the very bottom black bar text (with copyright) the text is not aligned centrally for some reason.

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HTML:

    <div class="copyright">
        Copyright &copy; 2011 Scirra.com.  All rights reserved.
    </div>
</body>
</html>

CSS:

.copyright
{
    color: white;
    text-align:center;
    background-color: #000000;
    font-size: 12px;
    padding: 3px;
}

Edit

Answer solves the alignment, but firefox only has the horizontal scroll bug:

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Add overflow: hidden; to your .footerWrapper class. The -20px margin-bottom is causing the shift, and setting its overflow to hidden fixes it:

[Edit] Larger version of image

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You are using Mozilla's box-shadow CSS and this is causing the horizontal scroll bar. It is not a bug but is causing the issue.