Add-ons are themes that change the way Thunderbird looks or extensions that add functionality or change the way Thunderbird works. You can check out many of the add-ons available for Thunderbird at addons.thunderbird.net.
"MailExtensions" are WebExtensions with some added features specific to Thunderbird. WebExtensions is the current type of Extension used for Firefox and Chrome. Unlike how extensions once were, MailExtensions don't have complete access to Thunderbird's internal components and UI, but instead are able to access functionality through the use of APIs.
The extensions that were used before MailExtensions are now referred to as "Legacy Extensions" and fall into two categories:
The original type of extension for Thunderbird and Firefox. This type uses documents that overlay Thunderbird UI, adding and modifying it. Uses an RDF manifest (
install.rdf) and requires a restart of Thunderbird for installation/uninstallation, upgrading/downgrading and enabling/disabling.
Uses a bootstrap file (
bootstrap.js) as an entry point to the extension. The file defines four methods (
shutdown) from which all extension behaviour is controlled. This type of extension can be installed or shut down without restarting Thunderbird, so it’s sometimes called a “restartless” extension. Also uses an RDF manifest (
If you are currently maintaining a legacy extension please consider checking out our guides on updating extensions for Thunderbird 68 and Thunderbird 78:
Themes change the way that Thunderbird looks, for instance - here is a screenshot of the side panel using the built-in dark theme:
Static themes, like the name implies - are static and do not change. They have a set color or images that make up the theme and this does not change.
Dynamic themes use the theme API to do more interesting things with the look of Thunderbird. For instance, one example used in Firefox documentation is a dynamic theme that changes the theme colors based on the time of day.
We've begun to assemble a collection of tips and tricks for making add-on development easier: