After nine years of work, the last UI components that used the XUL namespace have been removed from the Firefox codebase. Thus, with a few exceptions, Firefox's UI is now rendered using conventional web technologies (mostly CSS flexbox) rather than XUL-specific handlers (-moz-box, -moz-inline-box, -moz-grid, - moz-stack, -moz-popup). As an exception, XUL continues to be used to display system menus and popup panels ( And ), but in the future they plan to use the Popover API for similar functionality.
The ability to use XUL in add-ons was discontinued in 2017, and the interface got rid of XBL (XML Binding Language, XUL extension) bindings in 2019 (XBL bindings that define the behavior of XUL widgets were replaced with Web Components), but however, XUL handlers continued to be used in the formation of browser interface elements.
Source: opennet.ru