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.
This page is out of date: now the SDK ships inside Firefox, compatibility between the SDK modules and Firefox versions is no longer an issue.