The add-on ecosystem of Chrome currently dwarfs Firefoxes. Firefox extensions used to be more capable, but there's just more and certainly more NEW ones for Chrome.
(I'm sure this was an important consideration for Mozilla as well - it reduced the supposed value of their "more powerful" XUL extension system, as in practice you can often find a Chrome extension to do what you want, but not a Firefox one)
But it can't keep XUL compatibility and do major overhauls of the browser.
If XUL compatibility doesn't even get you a clear advantage in the add-ons area, should it block modernizing the browser?
Firefox can't be the browser that people use only because they're stuck on it because they have extensions that changes how the browser works. Think about that for a bit.
(I'm sure this was an important consideration for Mozilla as well - it reduced the supposed value of their "more powerful" XUL extension system, as in practice you can often find a Chrome extension to do what you want, but not a Firefox one)