You can try adding this to your CSS footer (in addition to existing footer styles).
footer { display:block; }
This fixed this for me under FF3.6, and I assume that this will fix it in IE as well. The default display style for <footer> is inline , which does not play well with backgrounds, regardless of browser. Applying a block to it will refer to it more like a div.
You will also want to be careful using HTML 5. Not sure how well it is supported in all browsers. You might want to consider <div id="footer"><!-- contents of footer --></div> and apply styles to it using id in CSS.
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