Our site is available via plain HTTP (default) and SSL (optional),
but the plugin is using SSL URIs in the sitemap when it should be
using plain HTTP. This is especially problematic since it then
specifies an SSL URI for the sitemap.xsl even when loading the map
over plain HTTP resulting in an error; Firefox reports "Error loading stylesheet: An unknown error has occurred (805303f4)" and then gives the SSL URI of the stylesheet.
A work-around is to disable the "Use default" XSLT option and supply a relative URI to the default (or other) XSLT i.e; /wp-content/plugins/google-sitemap-generator/sitemap.xsl