It still is. The entire Firefox UI exists in the DOM -- the URL bar is an HTML input, the toolbar buttons are XUL <toolbarbutton>s. The box model can even be us

It still is. The entire Firefox UI exists in the DOM -- the URL bar is an HTML input, the toolbar buttons are XUL <toolbarbutton>s. The box model can even be used from normal web pages! Just add this CSS to an element: display: -moz-box; and then you can -moz-box-flex it to your heart's content. The box model will no longer function once the patches that this blog post describes make it into a release, but XUL elements themselves are still around.