XBL

[3][4] However, the UXP fork of Firefox intends to continue supporting XBL indefinitely.

[5] Mozilla attempted to standardize XBL 2.0 via W3C in 2007,[6] but due to lack of interest from other web browser vendors, abandoned it in 2012.

[7] The Shadow DOM specification acknowledges XBL as a strong influence.

But XBL is needed to alter the behavior of a XUL widget, such as a scroll bar.

An XBL file contains bindings, each of which describes the behavior of a XUL widget.