Yahoo! Mail Alerter by Ngo Kim Phu

Alerts you when new emails arrived in your Yahoo! Mail box.
Uses cookie to make direct connection to Yahoo! Mail, i.e. does not require password, and your data fetched from Yahoo! Mail server doesn't go to any remote server.
Now that this add-on works even if you enable tracker content blocking (Trackers or Third-party tracker cookies), consider enabling those features.

Go to options to change mail check frequency or turn off/on notification.

Yahoo! and Yahoo! Mail are trademarks of ‎Oath Inc., with which this add-on is not affiliated.