I'm definitely happy with the new UI changes coming in Firefox 57. They're unobtrusive, functional, and elegant -- for someone who picks a web browser based on (a) power-user features, and (b) what the top strip with the tab bar and tool bar looks like, Firefox has leapfrogged past Chrome in a lot of respects.
It's too bad they had to drop XUL plugins to refresh their UI, though. Many of the most useful plugins were XUL plugins that were able to modify the UI in interesting ways, and tab/session management, in particular, suffers for it. The Tab Groups extension was excellent, and has no parallel in Chrome or other WebExtension-based browsers.
At the same time, I hope the new UI will eventually reduce the need for such extensions in the first place. Vivaldi has taken a stab at this with their own tab groups and putting tabs on the side; both of these are super useful for dealing with lots of tabs (although a little awkward in various ways). I hope Firefox will follow suit, or at minimum, provide more APIs to allow extensions to better-manage browser tabs and experiment with different philosophies.