A sidebar is a pane that is displayed at the left-hand side of the browser window, next to the web page. The browser provides a UI that enables the user to see the currently available sidebars and to select a sidebar to display.
The sidebar_action key enables you to define the default properties for the sidebar. You can change these properties at runtime using the sidebarAction API.
Syntax
The sidebar_action key is an object that may have any of the properties listed below. The only mandatory property is default_panel.
Use this to include a stylesheet in your popup that will make it look consistent with the browser's UI and with other extensions that use the browser_style property.
In Firefox, the stylesheet can be seen at chrome://browser/content/extension.css, or chrome://browser/content/extension-mac.css on macOS. When setting dimensions, be aware that this style sheet currently sets box-sizing: border-box (see box-sizing).
The Firefox Style Guide describes the classes you can apply to elements in the sidebar in order to get particular styles.
default_icon
Object or String
Use this to specify one or more icons for the sidebar. The icon is shown in the browser's UI for opening and closing sidebars.
Icons are specified as URLs relative to the manifest.json file itself.
You can specify a single icon file by supplying a string here:
"default_icon":"path/to/geo.svg"
To specify multiple icons in different sizes, specify an object here. The name of each property is the icon's height in pixels, and must be convertible to an integer. The value is the URL. For example:
This property is optional: if it is omitted, the sidebar doesn't get an icon.
default_panel
String
The path to an HTML file that specifies the sidebar's contents.
The HTML file may include CSS and JavaScript files using <link> and <script> elements, just like a normal web page.
Unlike a normal web page, JavaScript running in the panel can access all the WebExtension APIs (subject, of course, to the extension having the appropriate permissions).
Optional, defaulting to true. Determines whether the sidebar should open on install. The default behavior is to open the sidebar when installation is completed.