Colorful Blur by tomp2

Colorful Blur lets you set a different color theme with a blurred background image for each Firefox window.

There are 8 color presets: Original (pink/purple), Purple, Red, Orange, Yellow, Green, Cyan, and Blue. You can set a different color for each open window. The toolbar popup shows all your windows and you can change the color by clicking a color dot. It is also possible to drag the dots to change their order. When you open a new window, it gets a different color automatically, so it is easier to keep track of your windows.

The extension tries its best to remember the color for each window so that they would stay when you reopen Firefox.