My FF flat and simple (as of 9 Jul 16)

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

#navigator-toolbox:after {
    height: 0 !important;
}

browser {
  background-color: #000 !important;
}
#titlebar-content {
    background-color:#080808 !important;
}
#titlebar-close {
    list-style-image: url(chrome://browser/skin/caption-buttons.svg#close-white) !important;
}
#titlebar-min {
    list-style-image: url(chrome://browser/skin/caption-buttons.svg#minimize-white) !important;
}
#titlebar-max {
    list-style-image: url(chrome://browser/skin/caption-buttons.svg#restore-white) !important;
}
tab[selected="true"] {
   color:#fff !important;
}
.tabbrowser-tab:not([selected="true"]) {
    color:#6c6c6c !important;
}
#navigator-toolbox:after {
    height: 0 !important;
}
#nav-bar,
#TabsToolbar,
#PersonalToolbar {
  background: #000 !important;
  border: 0px !important;
}
.toolbarbutton-text {
color:#6a6a6a !important;
}
.toolbarbutton-1 {
border-bottom: 1px solid transparent !important;
}
findbar {
    -moz-appearance: none !important;
    background-color: #000 !important;
}
#bookmarks-view,
#historyTree,
#history-panel,
#sidebar-header {
    background-color: #000!important;
    color: #ccc!important;
}
menuitem,
menu {
    background-color: #000 !important;
    color: #ccc !important;
    border: none !important;
}
menuseparator {
    display:none !important;
}
menupopup {
    background-color: #000 !important;
    -moz-appearance: none!important;
    border:1px solid #000 !important;
}
menupopup > menu:hover {
    background-color: #111 !important;
}

menuitem:hover {
    background-color: #111 !important;
    color: #fff !important;
}
arrowscrollbox {
    background-color: #000!important;
    color: #ccc!important;
}
.statuspanel-label {
    color: #ccc !important;
    background: #000 !important;
    border: none !important;
}
#main-menubar, #nav-bar {
    box-shadow: none !important;
}
.tab-background-middle[selected=true]{
    background:#000 !important;
}
.tab-background-start[selected=true]::before{
    clip-path: url(chrome://browser/content/browser.xul#tab-curve-clip-path-start)!important;
}
.tab-background-end[selected=true]::before{
    clip-path: url(chrome://browser/content/browser.xul#tab-curve-clip-path-end)!important;
}
.tab-background .tab-background-start[selected=true]::before,.tab-background  .tab-background-end[selected=true]::before{
    background:#000 !important;
}
.tab-background-start[selected=true]:-moz-locale-dir(ltr)::after,
.tab-background-end[selected=true]:-moz-locale-dir(ltr)::after {
    background:transparent !important;
}
#urlbar {
    background-color:#111 !important;
    border:none !important;
    color:#aaa !important;
}
.searchbar-textbox {
    background-color:#111 !important;
    border:none !important;
    color:#aaa !important;
}
#titlebar-close, #titlebar-max, #titlebar-min {
    background-color:#080808 !important;
}
#titlebar-close:hover, #titlebar-max:hover, #titlebar-min:hover {
    background-color:#222 !important;
}
.tab-background {
    background:transparent !important;
}
.tabs-newtab-button:hover {
   background-image: url('data:image/png;base64,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'),  url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAYAAAAfCAYAAADEFbwbAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsMAAA7DAcdvqGQAAAAYdEVYdFNvZnR3YXJlAHBhaW50Lm5ldCA0LjAuOWwzfk4AAAAeSURBVDhPY5CWlsaKsQqCMIiYiQ2PSoxKDE0J6ZkA6GKi2TsIuu8AAAAASUVORK5CYII='),  url('data:image/png;base64,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')
 !important;
}
#forward-button#forward-button#forward-button>image.toolbarbutton-icon,
#back-button#back-button#back-button>image.toolbarbutton-icon
{
border-color: #111 !important;
border:1px solid #333 !important;
}
#back-button, #forward-button {
   border-bottom:none !important;
}
.tabbrowser-tab:hover:not([selected="true"]) {
    color: #fff !important;
}
menupopup>hbox
{
   border:0px !important;
}
/* AGENT_SHEET */

scrollbar[orient='vertical'] > scrollbarbutton {
    color:#000 !important;
    background-color: #1a1a1a  !important;
    border:none !important;
    min-width: 10px !important;
    max-width: 10px !important;
}
scrollbar[orient='horizontal'] > scrollbarbutton {
    color:#000 !important;
    background-color: #1a1a1a  !important;
    border:none !important;
    min-height: 10px !important;
    max-height: 10px !important;
}
scrollbar > slider, scrollbar > slider > thumb, scrollbar > scrollbarbutton{
    -moz-appearance: none !important;
}
scrollbar > slider {
    border-width: 0px;
    background-color: #1a1a1a;
}
scrollbar > slider > thumb {
    border-width: 0px;
    background-color: #000;
}
scrollbar > slider > thumb:hover {
    background-color: #080808;
}
.toolbarbutton-1:hover {
border-bottom: 1px solid #fff !important;
}
.tab-background-middle[selected=true]{
    border-bottom:2px solid #333 !important;
}
.tab-background-middle:hover{
    border-bottom:2px solid #fff !important;
}