Solution: This appears to be a bug with Google Analytics and Firefox (3.6.3 for me). If you use the Analytics overlay feature to view a site, then the next time you try to view the site in Firefox, you have this problem. To fix it, simply delete all your cookies. Or if you can find it, only delete the analytics cookies.
Note: If you look at your code in Firebug, you will see Analytics has added this code to your page.
<div id="ga_shade" style="background-color: #eeeeee; display: block; height: 1000px; left: 0px; opacity: 0.5; position: absolute; top: 0px; width: 100%; z-index: 100000;"></div>
Removing the cookie removes the above HTML code from your page.