Adding DuckDuckGo as a search engine option

1. In the menu bar, select Help.
2. Select Troubleshooting Information.
3. Select Show Folder.
4. In the folder, create a folder called searchplugins if it doesn't exist.
5. Save this file in the searchplugins folder.
6. Restart Thunderbird.

Making DuckDuckGo the default search engine option

1. Add DuckDuckGo as a search engine option. (see the instructions above)
2. In the search bar, type something and select Search [your current default Thunderbird search engine] for:
3. Click the DuckDuckGo icon on the left.
4. Click the heart icon on the bottom left.

Thanks to Andres Muñiz for locating these instructions.