That's technically correct, which is the best kind of correct, but horribly misleading.
Firefox has always been mostly written in JavaScript, but not HTML [1]. A bunch of features that are being standardized now, like Web Components [2], are pretty similar to stuff that Firefox has used in non-standard form for decades.
2. Even if Firefox were still 100% XUL today, it wouldn't matter. The context here is the use of JS as a general applications programming language and a signpost addressing the uninitiated who haven't been "following JS". Whether it's touching DOM nodes that are in the XUL namespace or the (X)HTML one (or whether it involves DOM nodes at all) is orthogonal. Bringing this up is the kind of change of subject that constitutes misdirection.
3. I don't know what you think the role of Web Components being like XBL plays in this conversation, but it strengthens the underlying point; it doesn't weaken it...
Overall, this is a very odd response, to be generous. More accurately, it's horribly misleading to label my comment "technically correct[...] but horribly misleading".
Mostly, they're DOM enhancers, and Firefox has always been coded against its own internally-maintained, enhanced version of the DOM.
So, again, my comments were not misleading. And these remarks don't provide any information that contradicts the claims I made. (You are, though, calling attention to what is actually misleading here—and tacitly supporting what was my argument all along, without seeming to realize it.)