In an ideal world, all the add-ons you would want are officially vetted, approved, and can be found on the official website, .įor add-ons that fall into this category, you can simply go to the Add-ons Manager tab in Thunderbird by clicking the menu button followed by Add-ons and click on the Extensions panel on the left to display the search box in the top-right corner. 2 A slightly less ideal case (Install from a downloaded XPI file).