After nine years of operation, the last components for rendering the Firefox interface that used the XUL namespace have been removed from the codebase. Consequently, with a few exceptions, common web technologies (mainly CSS flexbox) are now used for the Firefox user interface rendering, rather than specific XUL handlers (-moz-box, -moz-inline-box, -moz-grid, -moz-stack, -moz-popup). As an exception, XUL continues to be used for displaying system menus and pop-up panels (<xul:menupopup> and <xul:panel>), but in the future, Popover API is planned to be utilized for such functionality.
The ability to use XUL in extensions was discontinued in 2017, and the interface was freed from XBL (XML Binding Language, an extension of XUL) bindings in 2019 (XBL bindings, which defined the behavior of XUL widgets, were replaced with Web Components). However, XUL handlers continued to be utilized in the formation of browser interface elements.
Source: opennet.ru
