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.
Aero was probably the last time I ever saw a new interface style and thought it looked cool. Now it’s just endless reshuffling of parameters. Maybe we just arrived at the ideal set of parameters to reshuffle and any major change at this point could only make things worse.