Mozilla Developers about successful work to remove language components from the Firefox code (XML Binding Language). During the work, which since 2017, about 300 different bindings using XBL have been removed from the code, and about 40 thousand lines of code have been rewritten. These components have been replaced with analogues based on written using conventional web technologies.
XBL was used to organize the Firefox interface and allow you to create bindings that change the behavior of widgets in the XUL language. In 2017, Mozilla deprecated XBL and XUL and stopped supporting add-ons written using these technologies in Firefox 57. In the same time to rewrite XBL/XUL-bound Firefox components. The last interface components tied to XBL were the address bar and add-on manager, which were replaced with new implementations in Firefox 68.
Source: opennet.ru