XBL is a declarative XML language which implements the publish/subscribe pattern. It is used to build extension for XUL applications, including the Firefox browser and the Komodo editor.
XML Binding Language (XBL, sometimes also called Extensible Bindings Language) is a language for describing bindings that can be attached to elements in other documents. The element that the binding is attached to, called the bound element, acquires the new behavior specified by the binding.