You can make changes to the Firefox UI with CSS coding yourself: https://www.userchrome.org/ For example, my Firefox looks currently like this (looks pretty much the same in a non-private session too), just like my operating system with KDE:

It's a complex mix of addons, custom CSS, custom JavaScrip, some custom settings in Firefox and so on. You can copy CSS codes into your userChrome.css file, some good sources I use are:
- CSS for userChrome.css: https://github.com/MrOtherGuy/firefox-csshacks
- JavaScript for user.js and prefs.js: https://github.com/yokoffing/Betterfox