The Firefox UI is now built with Web Components

A couple of weeks ago, we landed a commit that took years of effort at Mozilla. It removed "XBL", which means we've completed the process of migrating the Firefox UI to Web Components. It wasn't easy - but I'll get to that later. It's taken a couple years of work of remarkably steady progress by a small team of engineers along with the support of the rest of the organization, and I'm happy to report that we've now finished. This is a big accomplishment on its own, and also a foundational improvement for Firefox. It allows teams to focus efforts on modern web standards, and means we can remove a whole lot of duplicated and complicated functionality that wasn't exposed to websites. The fact the people at Mozilla have been able to do this without any major disruptions to Firefox users is pretty impressive.