I still have Fennec (Firefox from F-Droid) installed, I didn't notice any major changes since I stopped using it. I use it when there is some web application that expects a modern browser and the webview can't handle it.
But then, I also didn't think 57 for desktop was a big update. For me, it made zero speed difference (Firefox has always been fast on my laptop), on Linux with vertical tabs there is almost zero UI difference, and the only new thing was having to throw out all add-ons with the bathwater. Somehow, it seems I always experience things differently from other people.
Experiences from years ago aren't really relevant any more, as Firefox has gotten a lot of work done the last couple of years.