The issue is related to browser support for client-side XSLT, and Firefox's preferences for displaying RSS. Firefox is pushy when it comes to handling and rendering RSS feeds; it inserts its own stylsheet, overriding the one that feedburner offers. I think you can override that in your browser options, but it may not be worth the hassle, and that only solves the problem for you, not for your audience.
But is it really a problem? Don't fret, your feed isn't broken or anything - it's just being shown in a different way. RSS is not intended to be styled up like a web page anyhow.
The Mozilla Developer Center recommends:
Recent browsers (Firefox 2, Safari (version?), IE 7) will display RSS content specially, rather than applying custom style sheets to the XML data. While this is what the user and site author want in the vast majority of cases, you may wish to override this behavior for a specific case. Your choices include: