Mozilla developer Brian Grinstead announced that the Firefox UI is now built by Web Components, removing XBL. Firefox UI was originally built with DOM and JS, and more than 20 years ago they were bold new technologies for native applications.
Thus, Before the Web platform did not support them, Mozilla implemented some of the features required to build complex web applications. These features are slowly evolving into standard specifications such as CSS fleXbox and Web Components.
When all this happens, it’s certainly the easiest thing to allow an existing code base to use the original version and require the platform to support both, after all, it’s hard or not cost-effective to rewrite the old code.
But Firefox decided to implement Web Components and port existing UI components in a gradual manner that took two years to complete. This achievement will allow Mozilla to focus on modern Web standards, removing a large number of repetitive and complex features.