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.
Speaking as someone who’s colorblind, I rely heavily on shapes, straight borders, and strongly contrasting colors to be able to tell the difference between objects and the boundaries between objects.
Nice that Firefox is throwing all colorblind people under the bus(~5% of the population) in the same way Apple has been doing for years.