The Add-on Builder was a web-based development environment that allowed developers to create add-ons using the SDK APIs, but without needing to use the
cfx command line tool. It was retired on April 1, 2014, and the "builder.addons.mozilla.org" domain now redirects to this page.
If you have only used the SDK through the Builder, you already know most of what you need to know to develop using just the SDK. The high-level and low-level APIs used for Builder add-ons are exactly the same for Builder and SDK. To switch to the SDK: