Change Default Feed Reader in Firefox - How To

Managing Feed Readers in Firefox
When Firefox has a default feed reader configured, the ability to preview a feed prior to subscription is lost. This can present a challenge when you wish to quickly review feed content without immediately adding it to your chosen reader.
For users unfamiliar with Firefox’s RSS functionality, observe the orange RSS icon located in the address bar. Clicking this icon will direct you to a preview page displaying the feed’s contents.
It’s important to note that this preview page doesn’t showcase the complete feed; rather, it presents excerpts from each post. You will also find a checkbox labeled "Always use Google Reader to subscribe to feeds" on this page.
Supported Feed Readers
Firefox extends support to other feed readers beyond Google Reader, including Bloglines and Yahoo.
Selecting this checkbox establishes the default feed reader upon initial configuration. Should you need to modify this setting later, navigate to Tools, then Options, and select the Feeds tab.
Within the Feeds tab, you can designate your preferred application for handling feeds. To revert to the preview option, select the radio button indicating "Show me a preview and ask me which Feed Reader to use".
Recommendation
For dedicated RSS users, utilizing Google Reader is highly recommended.