Not a fan of the new look. If you’re in the same boat, plug:
about:config
into your URL bar, and search for:
browser.nova.enabled
Toggle it to false using the button to the right.
There are functional changes that Nova also introduced: this won’t effect those - only the aesthetic.
The new UI is fine for me, but it’s also nice to see they kept the nova.enabled flag after the update for those who don’t want it.
The best change for me was moving compact tabs from an about:config flag to an actual setting. Was using that before and it’s nice to see it’s not going away after all.