Well, or Firefox as a web browser has to render HTML&#x2F;CSS&#x2F;JavaScript no matter what, and now that HTML&#x2F;CSS is at feature parity or better with XUL in the space XUL is meant to occupy, it doesn&#x27;t make sense for Mozilla to maintain two competing technologies when one receives 95% of their internal developer attention and 99.99999999999999% of external developer attention.