You're right - but the real win here is that hundred of thousands of developers already know HTML + CSS, and very few developers know XUL or even any native UI API. Given that these native APIs change with the platform, many developers have given up on them since HTML + CSS will be useful for the foreseeable future.

Is XUL really better than HTML + CSS? Probably. But HTML + CSS is getting a lot better (flexbox, etc), and tons of developers know it now and can contribute. Plus, dogfooding. I think it's a really good move!

Sure, RAM usage will probably get worse, but perhaps it will motivate them to improve that as well, and all users of the browser on all pages will benefit as a result.