There is help on the 'net that tells you to download and install an xml file. You were supposed to put it into a folder in your profile named searchplugins. I belive this is what was referred to as opensearch. AFAIK this stopped working a few versions back. A workaround to to use the ''Google Search'' add-on which provides its own searchplugins folder, preloaded, naturally, with a google.xml file. You can add duckduckgo's xml file to this folder. https://addons.mozilla.org/en-GB/thunderbird/addon/google-search-for-thunderbi/ I have used startpage.com but I don't recall if they offered a ready-to-use xml file. I'd suggest you check startpage's help on their website.

http://forums.mozillazine.org/viewtopic.php?p=14688149#p14688149 http://bedroomcoders.co.uk/adding-startpage-com-search-engine-to-thunderbird/