Thunderbird is starting to provide an API similar to Firefox's WebExtensions. There is overlapping API for commonly used elements like windows and tabs, but there will also be mail-specific APIs to provide access to E-Mails and Thunderbird-specific features like the Cloudfile Providers.

This repository serves as a starting point for experimenting with new APIs before we push them to nightly builds, and should also serve as examples for add-on developers wanting to create their own APIs.

The following WebExtension Experiments are available in this repository for Thunderbird. Note that WebExtensions experiments are supported starting Thunderbird 63. It is recommended to use a nightly build when testing and developing WebExtension Experiments.

We would like to encourage add-on developers to contribute their APIs, so they can become part of core Thunderbird. Unlike Firefox, the team behind Thunderbird is mostly volunteer driven. Therefore, we need your support to provide add-on developers with APIs that provide access to Thunderbird's features.

When creating a new API, please start with the Firefox documentation on WebExtension Experiments. Support for WebExtensions is provided by the Mozilla Platform, therefore the basics are exactly the same. You can also read some of the code specific to Thunderbird WebExtensions, which will provide further examples.

If you have an idea for an API that you think would be worthwhile, please file an issue on this repository. We are open to general ideas, but if you have a suggestion for specific API methods and events we would also enjoy your input. Please make sure to read the design principles when filing a new issue.

WebExtension APIs are meant to provide a stable API for add-on developers to use, while at the same time hiding the details and allowing the Thunderbird team to clean up and change the underlying code. The following principles should be kept in mind when devising new APIs:

• The APIs should not expose Thunderbird features that are too specific, or subject to change. Providing WebExtension APIs is a long term commitment.

• The APIs should be very high level. We are looking for "access to address books and cards", not "access to the xpcom address book service and its technical properties".

• APIs should not rely on technical implementation details of the underlying XPCOM components. There should be no magic numbers (use constant ls and enums), and try to anticipate how future additions would be least disruptive to the existing API.

• The UI exposed to WebExtensions should be limited to properties we can support long term, and are, if possible, easy to separate. A sidebar or toolbar button is perfect, adding arbitrary content to the message header not so much.

• To extend the previous point, we'd like to be careful when replacing built-in UI. A button in the quick search toolbar is great, but replacing the whole quick search bar with custom UI would need a special mechanism (like chrome_settings_override) and it should be evaluated if there isn't a better way to achieve the same result.

Generally it is a good idea to file an issue to discuss your API first. If you've already done so or would like to submit your API experiment for inclusion in this repository, please make sure you have completed the following:

• The API follows the design principles laid out in this README.
• The API code must be made available under the terms of the Mozilla Public License 2.0.
• The API code is contained in a sub directory and passes the eslint configuration.
• There is a reference to the API in this README file, along with a short description.

Each experiment should also contain this table to summarize common information. Not everything needs to be filled in from the start, but it will help over the course of the experiment.

Valid status values are:

• Draft: Initial commit, discussing design in linked github issue
• Accepted: API Experiment accepted for Thunderbird Core, bug is filed to integrate into comm-central
• Nightly: API Experiment landed in nightly builds, compatibility field should be "Since Thunderbird NN"
• Release: API Experiment is available in release builds, compatibility field should be "Since Thunderbird NN".

The compatibility field shows what versions the API Experiment was designed for. If you are working on it in nightly, please be sure to set/update the respective nightly version. If you are sure it also works for a range of nightlies, you can also add a range.

If you have questions about WebExtension Experiments in Thunderbird, please stop by our IRC channel #maildev on irc.mozilla.org. We are happy to tell you more about how they work, and engage in some ad-hoc discussion on how to design WebExtension APIs.

If you are migrating your legacy add-on to WebExtensions, you may need to rethink how your UI is presented, possibly in a way that seems less integrated. It may also mean that certain features will not be possible in the future. While this is unfortunate, we need to strike a balance between exposing all of the features the Mozilla Platform has to offer and making the APIs simple to maintain.

We are not doing this to limit creativity, but to ensure Thunderbird can thrive and add-ons are resilient to changes in Thunderbird and the Mozilla Platform. If you had a lot of trouble making your add-on compatible to Thunderbird 60, imagine it would just stay compatible despite far-reaching internal changes.