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:

• putting the <rss> content in a prefixed namespace
  
• preceding the <rss> element with an XML comment
  
• serving the content as UTF-16

source [developer.mozilla.org]

Thanks httpwebwitch for the info. I am keeping the feed as it is. Let the browsers handle the issue. As long as the feed is validated and the contents are displayed fine, I am OK with it.

Few more queries: How to display the full post content in Firefox? Its just showing up the post snippet which I dont want!

Also in Google Chrome the feed is not displayed properly! Any quick fix for that?