From Mozilla Firefox Help
Feeds are specially formatted documents which summarize the content of web
sites. For example, a feed might summarize the latest news headlines from a
news site or the latest posts on a blog. You can view the content of feeds in
Firefox, create Live Bookmarks for them, or add them to a feed reader
on your computer or on the Web. The optionspreferences in this pane determine what
Firefox does when you view a feed.
Show me a preview and ask me which Feed Reader to use
When you view a feed within Firefox, you will be shown a preview of
its contents. With this optionpreference selected, you are always given a
choice of what you would like to use to subscribe to the feed at the top of
the preview page.
Subscribe to the feed using
Instead of displaying a preview of the feed when you view one, you can have
the feed be opened directly in a feed reader by selecting a reader from the
list of available readers. You can choose to use an application on your
computer to subscribe to feeds by clicking the Choose Application...
button and finding the application on your computer. Alternately, you can
choose to automatically subscribe to feeds by saving them as Live Bookmarks.
To choose a subscription method, simply select it from the list.