The Firefox code has been completely purged of XBL

Mozilla developers informed about the successful completion of the work to remove from the Firefox code components in the XBL (XML Binding Language). During the work, which was ongoing started in 2017, about 300 different bindings using XBL were removed from the code, and approximately 40,000 lines of code were rewritten. The specified components were replaced with analogs based on Web Components, written using standard web technologies.

XBL was used to organize the interface of Firefox and allowed for creating bindings that changed the behavior of widgets in XUL. In 2017, Mozilla classified XBL and XUL as deprecated and ended support for extensions written using these technologies in Firefox 57. At the same time, work has begun work continued on rewriting components tied to XBL/XUL in Firefox. The last components of the interface tied to XBL were the address bar and the add-ons manager, which were replaced with new implementations in Firefox 68.

Source: opennet.ru