How do I change the scrollbar's color in Firefox?

My theme is Ambiance and I added

style "scrollbar" {
    bg[NORMAL]  = shade (0.9, @selected_bg_color)
    bg[SELECTED]    = shade (0.7, @selected_bg_color)
    bg[ACTIVE]  = shade (0.7, @selected_bg_color)
    bg[PRELIGHT]    = shade (0.7, @selected_bg_color)

    GtkScrollbar::trough-border = 2

    engine "murrine" {
        roundness   = 8
        border_shades   = {1.0, 1.0}

        contrast    = 0.0
    }
}

in ~/.theme/copy_of_the_Ambiance_theme/gtk-2.0/gtkrc after tweaking the shades and it worked.