Why would you need this when TB supports RSS feeds? Well - 1. TB does not support OPML import where the feeds are organized into folders. You will need to recreate your folder structure ... or use Forumzilla. 2. Why create your a new News account, when all you really want to do is to store feeds in your local folders?
Use along with the ThunderBrowse add-on for a smooth feed reading experience.
Drawbacks: It would be nice if Forumzilla could add feed urls to the mail.trusteddomains list automatically.
This is my most important extension, without it RSS is unusable in Thunderbird. The only thing I would add is being able to sort the columns in Feed Subscriptions, when managing a lot of feeds it can get confusing.