Use this API to register user scripts, third-party scripts designed to manipulate webpages or provide new features. Registering a user script instructs the browser to attach the script to pages that match the URL patterns specified during registration.

Note: This is documentation for the legacy API version, available in Firefox for Manifest V2. A new API has been designed, see WECG issue 279. This new version of the API will be available in Firefox for use in Manifest V3. Development is tracked in Firefox bug 1875475. Chrome includes an implementation of the new API. Meanwhile, when using Manifest V3 or higher, use scripting.registerContentScripts() to register scripts.

This API offers similar capabilities to contentScripts but with features suited to handling third-party scripts:

  • execution is in an isolated sandbox: each user script is run in an isolated sandbox within the web content processes, preventing accidental or deliberate interference among scripts.
  • access to the window and document global values related to the webpage the user script is attached to.
  • no access to WebExtension APIs or associated permissions granted to the extension: the API script, which inherits the extension's permissions, can provide packaged WebExtension APIs to registered user scripts. An API script is declared in the extension's manifest file using the "user_scripts" manifest key.

Warning: This API requires the presence of the user_scripts key in the manifest.json, even if no API script is specified. For example. user_scripts: {}.

To use the API, call register() passing in an object defining the scripts to register. The method returns a Promise that is resolved with a RegisteredUserScript object.

Note: User scripts are unregistered when the related extension page (from which the user scripts were registered) is unloaded, so you should register a user script from an extension page that persists at least as long as you want the user scripts to stay registered.



The object returned by the register() method. It represents the registered user scripts and is used to deregister the user scripts.



Registers user scripts.



An event available to the API script, registered in"user_scripts", that execute before a user script executes. Use it to trigger the export of the additional APIs provided by the API script, so they are available to the user script.

Browser compatibility

BCD tables only load in the browser

See also