Because it is required to properly display html from feeds with some browsers (not all).
In all cases, it's only a display issue in the xsl Transformed output, html is escaped in the xml source as it should, the RSS output is fully valid (look at the source code, it's xml).
You mean for the escaped html with ff ? I'm not very sure, it's working with IE on you site, and with FF and IE on phpBB SEO. Make sure you did not break any of the :