Add-ons using the techniques described in this document are considered a legacy technology in Firefox. 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.

Starting from Firefox 53, no new legacy add-ons will be accepted on addons.mozilla.org (AMO) for desktop Firefox and Firefox for Android.

Starting from Firefox 57, only extensions developed using WebExtensions APIs will be supported on Desktop Firefox and Firefox for Android.

Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to use WebExtensions APIs if they can. See the "Compatibility Milestones" document for more information.

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

Page Loading

On page load
How to execute code each time a new page is loaded in browser/mail
Progress Listeners
Progress listeners allow extensions to be notified of events associated with documents loading in the browser and with tab switching events.
Post data to window
Code to post data to an existing or new window/tab

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 Contributors to this page: wbamberg, bunnybooboo, DavidWalsh, Ptak82, Mgjbot, Taken, ericjung, Chbok
 Last updated by: wbamberg,