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.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
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 WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
- Classes and Inheritance
- Content Processes
- A content process was supposed to run all the code associated with a single tab. Conversely, an add-on process was supposed to run all the code associated with a single add-on. Neither content or add-on proceses were ever actually implemented, but by the time they were cancelled, the SDK was already designed with them in mind. To understand this article, it's probably best to read it as if content and add-on processes actually exist.
- Getting Started
- Learn how to contribute to the Add-on SDK.
- Private Properties