The Old Reader Notifier by Alexander Kashev

The Old Reader Notifier provides a toolbar icon to notify you about unread RSS items in your The Old Reader account.

Optionally, you can have context menu items to quickly subscribe to a page and desktop notifications about new items.

On the TOR website, this add-on provides a [b] keyboard shortcut to open an item in background.

Recommended extra steps:

1. Registering TOR as an RSS Feed Handler at the bottom of TOR Tour page.
2. Installing the Awesome RSS extension to easily detect feeds.

This extension is a port of the Chrome extension and is a direct FF57+ replacement of the old "The Old Reader Notifier" by Tomek Wasiak.