A couple weeks ago I posted an article that describes how you can use XML stylesheets with ROME to create RSS / Atom feeds that are ‘user friendly’ (like the ones that FeedBurner produces). Firefox 2.0, released just this past week, has a new feature which renders my article moot. The “Previewing and subscribing to Web feeds” feature, is a good one for most non technical users: click on a link that results in an RSS feed and Firefox will recognize the RSS content type and show you the RSS styled with their own style sheet even if the feed has provided a style sheet. The current workaround, should you want your visitors to see your style sheet instead of the default Firefox 2.0 stylesheet, is to
“…put in a comment ranting about the evils of sniffing web content and overriding the desires of web developers which is long enough to move “<rss” or “<feed” out of the first 512 bytes, since that’s all we sniff.”
(source)
There’s an extremely lively discussion going on in the mozilla.dev.apps.firefox newsgroup and on bugzilla about this behavior.