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.
If you only dislike the horrible AI-looking color scheme, you can install the Proton theme (by Firefox) from the settings.
In case you prefer a store link: https://addons.mozilla.org/en-US/firefox/addon/firefox-proton/
I honestly have no idea how anyone thought this disgusting mess was a good idea to roll out to everyone.