Mozilla released Firefox 57, also known as Firefox Quantum, in 2017. As part of this, Firefox finished transitioning to a multi-process architecture. Although XUL add-ons worked in this architecture, they created large performance problems. Because of this, Mozilla removed support for legacy add-ons, including the use of custom XUL code. Mozilla switched extensions over to Google's HTML5-based WebExtensions format. In 2017, Firefox still included 289 XUL bindings for native browser components. By the end of 2019, Mozilla had removed all XUL files from their mozilla-central codebase.
