The problem (or at least one of them) is that maintaining XUL and HTML is expensive. XUL, like HTML, has a bunch of weird corner cases, tricky layout behavior, complex interaction between elements, etc. The focus of the platform developers at Mozilla is obviously on the Web, so XUL has been in a crappy maintenance mode for a long time now.

Doing the Firefox chrome (heh) in HTML is another way to put our money where our mouth is, like the Firefox OS and Web App Runtime efforts.

There's a more detailed explanation of the situation of XUL here: https://lists.mozilla.org/pipermail/dev-platform/2014-Octobe...