Being a linux newbie, I haven’t had much luck finding how to configure Firefox to use an external RSS client. Here’s how:

You can go under Edit->Preferences and choose the Tab Feeds.
There you choose Subscribe to the feed using and click on the button Choose Application and choose /usr/bin/liferea-add-feed

Thanks to Ralph‘s instructions, I now have Liferea connected from Firefox on Ubuntu.

Creative Commons LicenseThe Add a feed to Liferea from Firefox by Simon Scullion, unless otherwise expressly stated, is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Spain License.