Well, RSS is just a schema for the encoding of syndicatable information - it doesn't give any display semantics at all. The reason it seems to work is that IE has a builtin transform to HTML for RSS feeds (as does Safari), so they convert the RSS into something more human friendly. Firefox, by default does not.
You can fix this by providing your own XSLT file that gives a specific RSS->XHTML transformation, so that all browsers will display the same result.