Firefox is my default browser (mainly because it's not based on Chromium, and Chromium is made by Google whose main job is to sell ads, so fuck them), but I don't like the new UI.

Quick steps to come back to the previous interface:

• install the theme firefox-proton: https://addons.mozilla.org/en-US/firefox/addon/firefox-proton/
• go to about:config (say you know what you're doing, blablabla...), look for browser.nova.enabled and change its value to false
• done