Recently added to the Firefox feature list: Live Bookmarks (or Livemarks). Livemarks weren't initially intended to make it into Firefox 1.0, as Ben Goodger commented when he filed bug 244078. It seems like they will make it anyway, and have already appeared in nightly branch builds since 2004-07-08.
Livemarks allow you to bookmark an RSS feed and these appear as bookmark folders, with individual items in the feed appearing as bookmarks. Just click on the "bookmark" and you will be taken to the page the item in the RSS feed is pointing to.
A lightning bolt icon appears on sites which have the <link rel="alternate" type="application/rss+xml" title="RSS" href="http://www.example.com/index.xml" /> bit in the <head> tags. All you got to do is to click on the icon and choose from the syndication formats available.
There's even a "Refresh Livemark" context menu option to keep your Livemarks up to date.
Not anything that would take me away from the current RSS aggregator I use, but the integration with the bookmarking system is a very intuitive model that would fit into most people's conceptual model of RSS feeds and bookmarks. I mean, an RSS feed is as good as a bookmark you keep somewhere (only it feeds you live content). RSS feed -> bookmark folder, RSS feed item -> bookmark. Gels nicely for me.
Get a nightly branch build if you're interested to see it in action for yourself.