The Onion Notifier di Stefan Sundin

There is a default database, but it can be edited by going to the addon options.

Suggest changes to the default database by filing an issue at: https://github.com/stefansundin/the-onion-notifier

To stop notifications for the duckduckgo search engine, consider installing this search engine that directly uses DDG's onion domain: https://cup.aihedy.com/firefox/addon/ddg-onion/