Sure, but that's only in index.xul, which is a very small part of the project. Everything else is plain old js+html (xhtml necessary for localization). Apologies, I didn't notice that when I said "there's no XUL in there".

As a meta-discussion, <vbox> is just a fancy way of saying:

div { display: -moz-box; -moz-box-orient: vertical; }

...which is the old flexbox implementation. But I digress :)

edit: link to the MDN article about `display: box`: https://developer.mozilla.org/en-US/docs/Web/CSS/box-orient