[Plugin: Google XML Sitemaps] Error building sitemap

Hi I am trying to install google XML Sitemaps 3.1.2 when I attempt to build the sitemap.xml I get the following error when I attempt to view it through firefox 3.0.5. IE7 works fine.

Error loading stylesheet: An XSLT stylesheet does not have an XML mimetype: https://www.vickitiede.com/wordpress/wp-content/plugins/google-sitemap-generator/sitemap.xsl

Any idea what is wrong?

Thanks