Your point that Firefox led site owners away from linking to the feed is an important one which I hadn't considered.
I also agree the icon's useful for indicating an existing feed. It's easy not to think about this use, because most of us probably just notice it when we need it but rarely click on it.
Other than the icon being an indicator, I don't find the way Firefox handles feeds too useful (that mix between bookmarks and news feeds).
You can easily tell firefox to use another program (or even service) to subscribe to the feeds. I've got mine going to google reader with them. The icon has been removed from firefox 4 (I'm running the betas). I don't know if they'll put it back in by default from any amount of pleading, BUT surprise surprise, there's an extension that reimplements previous functionality. https://addons.mozilla.org/en-US/firefox/addon/rss-icon/
Sites evolved away from having links to the feed in the HTML text BECAUSE browsers got this feature, led by Firefox.