Open Search enables Thunderbird users to search various websites from within Thunderbird and display the results in a Thunderbird tab. This article describes how to add additional websites to the Open Search interface.
The example below shows how to add Google to the website list. (Note that there is a Google Search for Thunderbird add-on that you can install rather than doing the steps described below. These steps are still potentially relevant however for installing other search engines.)
Create a directory called searchplugins if it does not already exist.
Click http://bwinton.latte.ca/Work/google.xml. Save the contents of the page to a file called google.xml in the searchplugins directory. (In Firefox, to save the page contents to a file, right-click on the page then select Save page as....)
Open Thunderbird. The Google icon should now be displayed in the website list on the Open Search page.