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.
You should avoid using the API if at all possible. We intend to deprecate it in future releases.
If you use this API you can expect your add-on to get an extra security review by addons.mozilla.org.
This module should not be confused with the "chrome" global variable that WebExtensions can use to access APIs.
chrome module gives an Add-on SDK add-on access to the Components object, which in turn gives it access to a large set of privileged low-level Firefox APIs.
You can see an example of using this API in the XUL Migration Guide.