How to change native (os x) browser window design (firefox)?

Question

User interface

With the help of stylish I edit the interface design.
How to change the native design of the browser frame - remove stripes on top + rounded edges? For example, like vivaldi, the angles are 90 degrees.
#main-window {
-moz-appearance: none !important;
}
-moz-appearance: - responsible for the appearance of the controls for the design of the axis.
Or, prevent the window from using this property - how?

Answer 1

Vivaldi's GUI is written in HTML5+JS+CSS, everything changes there. And in FF it's all compiled, sort of. at best, you can overlay a picture on top of the finished interface (the so-called browser themes)