I'm not sure why the div would appear if you didn't select Fixed Full Width footer from the drop down box, but its easy to fix. If you go to the bottom of your page and remove this line the fixed full width footer will be removed:
<div id="fixedFullWidthFooter">Fixed Full Width Footer</div>
edit: As Jason points out in the next post, it does appear to be a bug with choosing a different setting and then changing back to normal.