I spent a good deal of time dealing with XUL, heck I even helped with the French community. Designing UI was a ton more robust with it than with HTML, but XULrunner was (and still is from what I saw) not the most pleasant beast to run.<p>I&#x27;m not advocating a rewrite for the sake of novelty, but they&#x27;re part of a product life cycle.<p>You mentioned that it would mean dropping some features. I think that&#x27;s a good thing; an opportunity to cut the fat and make sure only what is proven and useful makes its way back into Firefox.<p>You seem to think volunteers won&#x27;t pick up the maintenance of Thunderbird; if so then why should Mozilla care about it?<p>Now, I don&#x27;t think HTML, CSS and JS are the best technologies. They each suck in their own way. But they are winning, and Mozilla is merely embracing that.