It auto-installed today. It seems to be faster than the last couple of 56 updates, which had been really irritating me by sucking up system resources. So far the biggest irritation I've found in 57.0 is WTF did they do with bookmarks? I may just need to get used to it, but they really mangled where all my bookmarks are. I've got to click like three times to get to what once was all in a dropdown menu.
Firefox on Mint Linux jumped to 57.0 yesterday or the day before (I forget).
Some things in the address bar area got moved to unexpected places. One of my add-ons survived. I have the menu bar enabled (for a more classic drop down menu interface), and that doesn't appear to have changed. So
my bookmarks are right where I expect them to be.
I'll miss the "Ant Video Downloader" add on, but there are alternatives, and they'll probably have a compatible version if I wait long enough.
The new tab page works differently. Hopefully it's more stable now.