# Supported Manifest Keys *Several manifest keys in the following table are common to Thunderbird and Firefox and link to MDN description pages. Please be aware, that MDN is dedicated to browsers and of course to Firefox. Some information listed on MDN may not apply to Thunderbird.* The two columns *MV2* and *MV3* specify whether the manifest key is supported in Manifest V2 and/or Manifest V3.

Manifest Key	MV2	MV3	Description
action	false	true	A browser action is a button that your extension can add to Thunderbird's main mailTab toolbar. The button has an icon, and may optionally have a popup whose content is specified using HTML, CSS, and JavaScript. Note: This key has been renamed from `browser_action` to `action` in Manifest V3.
applications	true	false	Deprecated. Use `browser_specific_settings` instead.
author	true	true	Defines the extension's author. If the `developer` key is supplied and it contains the `name` property, it will override the `author` key. There is no way to specify multiple authors. This is a localizable property.
background	true	true	Use the `background` key to include one or more background scripts, and optionally a background page in your extension. Background scripts are loaded as soon as the extension is loaded and stay loaded until the extension is disabled or uninstalled.
browser_action	true	false	A browser action is a button that your extension can add to Thunderbird's main mailTab toolbar. The button has an icon, and may optionally have a popup whose content is specified using HTML, CSS, and JavaScript. Note: This key has been renamed from `browser_action` to `action` in Manifest V3.
browser_specific_settings	true	true	Defines properties that are specific to a particular host application. Information for Thunderbird are stored in: `browser_specific_settings.gecko` More details can be found in our hello world example.
cloud_file	true	true	Defines a file link provider, which can be used to upload large attachments to a server, instead of attaching them directly to the email.
commands	true	true	Use the commands API to add keyboard shortcuts that trigger actions in your extension, for example, an action to open a browser action popup.
content_scripts	true	false	Instructs Thunderbird to load content scripts into web page tabs and windows, whose URL matches a given pattern.
content_security_policy	true	false	The default policy restricts the sources from which a content script can load `<script>` and `<object>` resources, and disallows potentially unsafe practices such as the use of `eval()`. See Default content security policy to learn more about the implications of this.
compose_action	true	true	A compose action is a button that your extension can add to the toolbar of Thunderbird's message compose tabs. The button has an icon, and may optionally have a popup whose content is specified using HTML, CSS, and JavaScript.
default_locale	true	true	This key must be present if the extension contains the `_locales` directory, and must be absent otherwise. It identifies a subdirectory of `_locales`, and this subdirectory will be used to find the default strings for your extension. See Internationalization.
description	true	true	Defines a short description of the extension, intended for display in the Add-ons Manager. This is a localizable property.
developer	true	true	Defines the name of the extension's developer and their homepage URL, intended for display in the add-on manager tab. The `name` and `url` properties, if present, will override the `author` and `homepage_url` keys, respectively. This is a localizable property.
dictionaries	true	true	The `dictionaries` key specifies the `locale_code` for which your extension supplies a dictionary.
homepage_url	true	true	URL for the extension's home page. If the `developer` key is supplied and it contains the `url` property, this will override the `homepage_url` key. This is a localizable property.
icons	true	true	The `icons` key specifies icons for your extension. Those icons will be used to represent the extension in components such as the Add-ons Manager.
manifest_version	true	true	This key specifies the manifest version used by this extension. Supported are `2` and `3` (since Thunderbird Beta 110).
message_display_action	true	true	A message display action is a button that your extension can add to the toolbar of Thunderbird's message display tabs. The button has an icon, and may optionally have a popup whose content is specified using HTML, CSS, and JavaScript.
name	true	true	Defines the name of the extension. This is used to identify the extension in the Add-on manager and on sites like addons.thunderbird.net.
optional_permissions	true	false	Defines permissions, which should be requested dynamically (when needed) and not during install.
options_ui	true	true	Defines an options page for your extension.
permissions	true	true	This key requests special powers for your extension. This key is an array of strings, and each string is a request for a permission.
protocol_handlers	true	true	This key registers one or more web-based protocol handlers. It allows to register a website or an extension page as a handler for a particular protocol. Note: The default click handler in Thunderbird web tabs is currently not working correctly with custom defined protocol handlers. It does work in WebExtension windows.
short_name	true	true	Short name for the extension. If given, this will be used in contexts where the name field is too long. It's recommended that the short name should not exceed 12 characters. If the short name field is not included in manifest.json, then name will be used instead and may be truncated. This is a localizable property.
theme	true	true	This key defines a static theme to be applied to Thunderbird.
theme_experiment	true	true	This key enables the definition of experimental `theme` key properties for the Thunderbird interface. These experiments are a precursor to proposing new theme features for inclusion in Thunderbird.
user_scripts	true	false	Instructs the browser to load a script packaged in the extension, known as the API script, this script is used to export a set of custom API methods for use in user scripts.
version	true	true	The version of the extension, formatted as numbers and ASCII characters separated by dots. For the details of the version format, see the Version format page.
web_accessible_resources	true	false	This key exposes selected extension resources (for example images, HTML, CSS or JavaScript) to web pages.

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