The users' perspective, in short: before 57, I had one browser that was fast but lacked all the UI tweaks that I had grown accustomed to over the course of a decade or so (Chrome/Chromium), and one browser that maybe wasn't so fast but had all those UI tweak (Firefox, which I used all the time).

Now I have not one but two fast browsers with a UI that I do not like.

It's perfectly understandable that Firefox devs would love Firefox to be more like Chrome, but that has nothing to do with what the (existing) users want: they already have Chrome available, they are not holding their breath waiting for Firefox to become Chrome.