Not sure if we really have this bug, haven't tested, but since somebody complained about this in a newsgroup this should be tested. A testcase would be an XML file with two stylesheets, second one having media print. According to the reporter, Mozilla always uses the last stylesheet, when it should use the first for screen and the second for print.
I will look into this issue. Not sure how we handles this in either xml or xhtml documents.
If I recall correctly, we _do_ parse the PI to get the media attribute and then pass that on to the CSS loader... A testcase would be nice.