- Adding a holiday calendar This article describes how to add a holiday calendar to the latest version of Lightning (a popular calendaring add-on for Thunderbird).
- Add Security Exception This article explains Thunderbird's communication security, and shows how to add an rule that exempts messages when the "'''Add Security Exception'''" dialog is displayed.
- Add Search Engine to Open Search Open Search enables Thunderbird users to search various websites from within Thunderbird. This article describes how to add additional websites to the Open Search interface.
- Add-Ons and Extensions FAQ Add-ons are extensions and other additions that add new functionality to Thunderbird or change its appearance. Thunderbird has an open, extensible design and program architecture. This enables people to create "add-ons" to Thunderbird.