It's an HTML-like language that defines the browser's interface, you can use it to change the shapes, positions, colors, whatever of your toolbars and tabs. Also they do still have customization via userChrome.css and I think you can re-enable XUL if you dig enough? It does get mixed a bit with HTML-namespaced tags too.
Pale Moon still supports XUL, if you want to play with it.
Among other things, it powered legacy extensions, and let them do far more than the essentially crippled WebExtensions
What even is XUL?
It's an HTML-like language that defines the browser's interface, you can use it to change the shapes, positions, colors, whatever of your toolbars and tabs. Also they do still have customization via userChrome.css and I think you can re-enable XUL if you dig enough? It does get mixed a bit with HTML-namespaced tags too.
Pale Moon still supports XUL, if you want to play with it.
Among other things, it powered legacy extensions, and let them do far more than the essentially crippled WebExtensions