“This means that (modulo a few exceptions documented below) all the Firefox UI is using regular web technology to render (mostly CSS flexbox).”
The bug to kill XUL layout was filed nine years ago. The final diff deletes 17,465 lines and adds 4,981, about 13k fewer lines for Mozilla’s layout team to maintain. XUL never supported basic CSS like proper absolute positioning, and it took hacks just to make a description element wrap. Firefox spent most of a decade paying interest on a layout engine nobody else used.