I have been having a problem like this as well. Here is what I did to get mine working in FF and IE
I changed this <?xml version="1.0" encoding="UTF-8"?>
to this `<?xml version=”1.0″ encoding=”UTF-8″?>
<?xml-stylesheet type=”text/xsl” href=”product-list.xslt”?>`
in the sitemap.xml file. THEN… I added this AddType application/xml .xsl to my .htaccess and now everything seems to be working fine for me.