The Firefox browser has removed XUL layout from its codebase, replacing it with regular web technology, mostly CSS flexbox. This change was a large effort, first proposed 9 years ago, and was made possible by the help of the front-end team and volunteers. The removal of XUL layout is expected to bring performance improvements and bugs that affect the web will now also affect the Firefox UI. The change also makes it easier for front-end developers to contribute to the Firefox UI, as they can now use regular CSS to change the UI or contribute to it.