> Could you check if there is a way to darken the menu of firefox (left-click one, and when opening a folder of favorites in the favorites bar)?

It's not possible to do this from an add-on. But I recommend you to 1) set Firefox theme to "Dark", 2) select dark theme in your OS (Windows: left Shift + left Alt + Print Screen, Linux: pick your favorite dark GTK 3 theme, for example, Adwaita Dark, macOS: you have to wait for Mojave).

UPD: the rest is rendered by your OS. So, see above.