API Implementation Basics

This page describes some of the pieces involved when creating WebExtension APIs. Detailed documentation about how these pieces work together to build specific features is in the next section.

The API Schema

As described previously, a WebExtension runs in a sandboxed environment but the implementation of a WebExtension API runs with full chrome privileges. API implementations do not directly interact with extensions’ Javascript environments, that is handled by the WebExtensions framework. Each API includes a schema that describes all the functions, events, and other properties that the API might inject into an extension’s Javascript environment. Among other things, the schema specifies the namespace into which an API should be injected, what permissions (if any) are required to use it, and in which contexts (e.g., extension pages, content scripts, etc) it should be available. The WebExtensions framework reads this schema and takes care of injecting the right objects into each extension Javascript environment.

API schemas are written in JSON and are based on JSON Schema with some extensions to describe API functions and events. The next section describes the format of the schema in detail.

The ExtensionAPI class

Every WebExtension API is represented by an instance of the Javascript ExtensionAPI class. An instance of its API class is created every time an extension that has access to the API is enabled. Instances of this class contain the implementations of functions and events that are exposed to extensions, and they also contain code for handling manifest keys as well as other part of the extension lifecycle (e.g., updates, uninstalls, etc.) The details of this class are covered in a subsequent section, for now the important point is that this class contains all the actual code that backs a particular WebExtension API.

Built-in APIs versus Experiments

A WebExtension API can be built directly into the browser or it can be contained in a special type of extension called a “WebExtension Experiment”. The API schema and the ExtensionAPI class are written in the same way regardless of how the API will be delivered, the rest of this section explains how to package a new API using these methods.

Adding a built-in API

Built-in WebExtension APIs are loaded lazily. That is, the schema and accompanying code are not actually loaded and interpreted until an extension that uses the API is activated. To actually register the API with the WebExtensions framework, an entry must be added to the list of WebExtensions modules in one of the following files:

  • toolkit/components/extensions/ext-toolkit.json
  • browser/components/extensions/ext-browser.json
  • mobile/android/components/extensions/ext-android.js (ugh android)

Here is a sample fragment for a new API:

"myapi": {
  "schema": "chrome://extensions/content/schemas/myapi.json",
  "url": "chrome://extensions/content/ext-myapi.js",
  "paths": [
    ["anothernamespace", "subproperty"]
  "scopes": ["addon_parent"],
  "permissions": ["myapi"],
  "manifest": ["myapi_key"],
  "events": ["update", "uninstall"]

The schema and url properties are simply URLs for the API schema and the code implementing the API. The chrome: URLs in the example above are typically created by adding entries to jar.mn in the mozilla-central directory where the API implementation is kept. The standard locations for API implementations are:

  • toolkit/components/extensions: This is where APIs that work in both the desktop and mobile versions of Firefox (as well as potentially any other applications built on Gecko) should go
  • browser/components/extensions: APIs that are only supported on Firefox for the desktop.
  • mobile/android/components/extensions: APIs that are only supported on Firefox for Android.

Within the appropriate extensions directory, the convention is that the API schema is in a file called schemas/name.json (where name is the name of the API, typically the same as its namespace if it has Javascript visible features). The code for the ExtensionAPI class is put in a file called ext-name.js. If the API has code that runs in a child process, that is conventionally put in a file called ext-c-name.js.

The remaining properties specify when an API should be loaded. The paths, scopes, and permissions properties together cause an API to be loaded when Javascript code in an extension references something beneath the browser global object that is part of the API. The paths property is an array of paths where each individual path is also an array of property names. In the example above, the sample API will be loaded if an extension references either browser.myapi or browser.anothernamespace.subproperty.

A reference to a property beneath browser only causes the API to be loaded if it occurs within a scope listed in the scopes property. A scope corresponds to the combination of a Javascript environment (e.g., extension pages, content scripts, etc) and the process in which the API code should run (which is either the main/parent process, or a content/child process). Valid scopes are:

  • "addon_parent", "addon_child: Extension pages
  • "content_parent", "content_child: Content scripts
  • "devtools_parent", "devtools_child": Devtools pages

The distinction between the _parent and _child scopes will be explained in further detail in following sections.

A reference to a property only causes the API to be loaded if the extension referencing the property also has all the permissions listed in the permissions property.

A WebExtension API that is controlled by a manifest key can also be loaded when an extension that includes the relevant manifest key is activated. This is specified by the manifest property, which lists any manifest keys that should cause the API to be loaded.

Finally, APIs can be loaded based on other events in the WebExtension lifecycle. These are listed in the events property and described in more detail in Managing the Extension Lifecycle.

WebExtensions Experiments

A new API may also be implemented within an extension. An API implemented this way is called a WebExtension Experiment. Experiments can be useful when actively developing a new API, as they do not require building Firefox locally. Note that extensions that include experiments cannot be signed by addons.mozilla.org. They may be installed temporarily via about:debugging or, on browser that support it (current Nightly and Developer Edition), by setting the preference xpinstall.signatures.required to false.

Experimental APIs have a few limitations compared with built-in APIs:

  • Experimental APIs can (currently) only be exposed to extension pages, not to devtools pages or to content scripts.
  • Experimental APIs cannot handle manifest keys (since the extension manifest needs to be parsed and validated before experimental APIs are loaded).
  • Experimental APIs cannot use the static "update" and "uninstall" lifecycle events (since in general those may occur when an affected extension is not active or installed).

Experimental APIs are declared in the experiment_apis property in a WebExtension’s manifest.json file. For example:

  "manifest_version": 2,
  "name": "Extension containing an experimental API",
  "experiment_apis": {
    "apiname": {
      "schema": "schema.json",
      "parent": {
        "scopes": ["addon_parent"],
        "paths": [["myapi"]],
        "script": "implementation.js"

      "child": {
        "scopes": ["addon_child"],
        "paths": [["myapi"]],
        "script" "child-implementation.js"

This is essentially the same information required for built-in APIs, just organized differently. The schema property is a relative path to a file inside the extension containing the API schema. The actual implementation details for the parent process and for child processes are defined in the parent and child properties of the API definition respectively. Inside these sections, the scope and paths properties have the same meaning as those properties in the definition of a built-in API (though see the note above about limitations; the only currently valid values for scope are "addon_parent" and "addon_child"). The script property is a relative path to a file inside the extension containing the implementation of the API.

The extension that includes an experiment defined in this way automatically gets access to the experimental API. An extension may also use an experimental API implemented in a different extension by including the string experiments.name in the permissions` property in its manifest.json file. In this case, the string name must be replace by the name of the API from the extension that defined it (e.g., apiname in the example above.