This page has the following CSS for div.photo elements:
div.photo { background-image: url(morning-coffee.jpg); background-repeat: no-repeat; } body.test-page div.photo { background-image: url(purple-horizon.jpg); background-repeat: no-repeat; width: 500px; height: 333px; }
Because the body element in this page has the "test-page" class, the div.photo element below should get the purple-horizon.jpg background image (and it does). However, if you load the page with Firebug's "Net" panel open, you can see that Firefox downloads both morning-coffee.jpg and purple-horizon.jpg.
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.3a1pre) Gecko/20090916 Minefield/3.7a1pre
Image credits: Morning Coffee by psd and Purple Horizon by Richard0. All images CC-Attribution licensed.
(This has been filed as Bug 517224)