I don't know, but who cares? What does anyone have against Firefox? It's fast, has a massive extensions library, open source, secure, private ... there's literally no reason whatsoever not to use it.
Blink has quite a few security benefits over Gecko, and while Firefox is plenty fast, Chromium is faster. I'm very much a Firefox gal, but there are plenty of reasons to use Chromium.
People don't like change. But I'm a hypocrite. I've tried switching to Firefox multiple times but I always go back, not even really sure why. I use it as a secondary browser these days.