I just stumbled across Koz’s “Feed Your Reader” extension for Firefox:
Feed Your Reader allows you to use the RSS Autodiscovery in firefox to launch external aggregators
The fact that Firefox lacks the ability to integrate with external news readers has been a major source of annoyance for me. Safari’s ‘Default RSS Reader’ preference is great and I’ve long wished Firefox offered something similar. Now, by the power vested in extensions, it does.
Check out Feed Your Reader. Word.

