XUL was a weight on developing for Firefox. It worked like HTML, but was nonstandard. At first, it meant they could quickly implement things like flexbox (&lt;hbox&gt; &#x2F; &lt;vbox&gt;) and gridlayout (&lt;grid&gt;) long before they were a thing in CSS.<p>But as HTML and CSS slowly got more features than XUL, XUL development slowed down, up to the point where writing the Firefox UI in XUL became a pain because of poor tooling and sneaky bugs. More and more pieces of Firefox got written in HTML inside XUL, and factorizing code between the pieces in XUL and those in HTML was nightmarish.<p>Dropping XUL means putting those bugs and issues behind us, and focusing development on a single DOM language. You would probably be surprised by how much of the UI already is in HTML; tab groups is almost all HTML, and the DevTools&#x27; editor and DOM inspector are in HTML as well.<p>As for donating to Mozilla, the distinction between Mozilla Corp and Mozilla Foundation is understandably complex for outsiders, but basically only Mozilla Corp makes money from the partnerships.