We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

Don't use these techniques to develop new add-ons. Use WebExtensions instead.

If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.


Enables you to read content from a URI.



readURI(uri, options)

Reads a URI and returns a promise.


uri : string
The URL, as a string, to load.

options : object
Optional options:

Name Type  
sync boolean

If this option is set to true, the promise returned will be resolved synchronously. Defaults to false if not provided.

charset string

The character set to use when read the content of the uri given. Defaults to UTF-8 if not provided.


promise : The promise that will be resolved with the content of the URL given.

Document Tags and Contributors

 Contributors to this page: wbamberg
 Last updated by: wbamberg,