Colorize Windows от Roli

Colorize Windows is a Firefox extension that allows you to change and customize browser Window colors. When you open a new window, it will have a distinct color. You can also change colors by clicking on the extension icon.

Colors are organized into families (Red, Orange, Yellow, Lime, Green, Teal, Blue, Indigo, Purple, Pink, Brown, Gray), so you can keep every window for one activity or project within the same family while still telling them apart by shade.