Firefox UI is very flexible, it's mainly just html+css+javascript at this point. But they have greatly removed support for changing through the UI itself over t

Firefox UI is very flexible, it's mainly just html+css+javascript at this point. But they have greatly removed support for changing through the UI itself over the years. So now you need to to change it through add-ons and hacks (userstyle/userchrome). Floorp seems to build mostly on those hacks and some internal addons of their own.