Firefox has it's own layout language called XUL that is used to style and design much of the UI inside Firefox. If HTML is so great why did Firefox need XUL? The reason is that HTML and CSS are inneficient at describing user itnerfaces, designer intent and basic design. Google Chrome's UI? Nope. Safari UI? Nope. Internet Explorer? Yeah Right.
Standards people, it's all about standards! Sure.
If the standards are so great and the browser makers claim great performance why aren't the core bits of browser user interfaces built using the very same tools?