If a website you frequently access has an RSS or Atom feed, you can make use of Firefox Live Bookmarks.
To add them, you need to set it up so Firefox is your default RSS reader — go to the Applications tab in Options, click “Web Feeds”, and pick your option.
To add one, you can click the Web Feed Button on the right side of the address bar. Then you choose where to put it — and ta-da!
More information is available here.