Hier zum Experimentieren meine userChrome.css mit der Beachtung, dass diese Code-Leichen hat, welche unter der Beta und "Dark Night Arcturus X" nicht mehr zum Tragen kommen, war nur zu faul aufzuräumen 
CSS
/* Tab-Line entfernen */
.tab-line[selected] {
display:none!important; }
/* Neue Mails hervorheben */
treechildren::-moz-tree-cell-text(new) {
color: #F3E2A9 ! important; }
/* Farbe Verfassen-Button */
#folderPaneWriteMessage {background: #25384A !important;}
/* Farbe selektierte Ordnerlisten */
treechildren::-moz-tree-row(selected) {
-moz-appearance: none !important;
background-color: #474749 !important;
border: none !important; }
treechildren::-moz-tree-row(selected, focus) {
-moz-appearance: none !important;
background-color: #474749 !important; }
treechildren::-moz-tree-row(selected, hover),
treechildren::-moz-tree-row(selected, focus, hover) {
background-color: #474749 !important; }
/* Farbe Kontenbezeichnung Ordner und Icon */
#folderTree > treechildren::-moz-tree-cell-text(isServer-true) {
font-family: New York !important;
font-size: 15px !important;
color: #F3E2A9 !important; }
.tabmail-tab[type="folder"][ServerType="imap"],
treechildren::-moz-tree-image(folderNameCol, isServer-true, serverType-imap)
{ moz-appearance:none !important; fill: #F3E2A9 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Inbox) {
color: #F3E2A9 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Junk) {
color: #FA8258 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Sent) {
color: #5FB404 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Trash) {
color: #A4A4A4 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Templates) {
color: #F5D0A9 !important; }
treechildren::-moz-tree-cell-text(folderNameCol, specialFolder-Drafts) {
color: #F4A460 !important; }
/* Farbe Lokale Ordner und Icon */
#folderTree > treechildren::-moz-tree-cell-text(folderNameCol, isServer-true, serverType-none) {
font-weight: bold !important;
font-family: New York !important;
font-size: 15px !important;
color: lightgrey !important; }
.tabmail-tab[type="folder"][ServerType="none"],
treechildren::-moz-tree-image(folderNameCol, isServer-true, serverType-none)
{ moz-appearance:none !important; fill: lightgrey !important; }
/* Farbe der Stausleiste */
#status-bar {
background: #323335 !important; }
/* Tab-Line entfernen */
.tab-line[selected] {
display:none!important; }
/* Schrift Überschrift Auswahl Ordnerliste */
/* ---Symbolleiste Konten- Ordneransicht Hoehe--- */
#folderPaneHeader {
max-height: 23px !important;
min-height: 18px !important;
border-bottom: 1px solid gray !important;}
/* Schrift Konten-Ordner-Liste */
#folderTree {
font-size: 16px !important;
font-family: Calibri !important; }
#folderTree > treechildren::-moz-tree-row {
height: 21px !important; }
/* Optimierung Nachrichten-Liste */
tr[is="thread-row"] {
appearance: none !important;
height: 21px !important;
font-family: Calbri !important;
font-size: 16px !important; }
#threadPaneHeaderBar.list-header-bar {display:none !important;}
#quickFilterBarContainer {display:none !important;}
#quick-filter-bar {display:none !important;}
/* Farbe "Zebramuster" Nachrichten-Liste */
#threadTree tr {
background-image: none !important;
background-color: #25384A !important;}
#threadTree tr:nth-child(2n) {
background-image: none !important;
background-color: #18181B !important;}
#threadTree tr.selected {
background-image: none !important;
background-color: #264765 !important;}
#threadTree tr:nth-child(2n).selected {
background-image: none !important;
background-color: #264765 !important;}
#threadTree tr:hover,
#threadTree tr.selected:hover,
#threadTree tr:nth-child(2n).selected:hover {
background-image: none !important;
background-color: #275C91 !important;}
/* Farbe "Zebramuster" Nachrichten-Liste
#threadTree treechildren::-moz-tree-row(even) {
background-image: none !important;
background-color: #4c4c51 !important;}
#threadTree treechildren::-moz-tree-row(selected) {
background-color: #316666 !important;
border: none !important} */
/* Farbe Einträge im Kontextmenü */
#mailContext-delete {
-moz-appearance: none !important;
color:red !important;}
#mailContext-delete:hover {
-moz-appearance: none !important;
background: darkred !important;
color: white !important;}
#mailContext-copyMenu {
-moz-appearance: none !important;
color:lightgreen !important;}
.folderMenuItem.menu-iconic[label="Lokale Ordner"] {
color: lightgreen !important;}
/* Farbe "Verfassen" Toolbar */
#button-send[disabled] {
background: gray !important;
color: darkgrey !important; }
#button-send:not([disabled]) {
background: green !important; }
#expand_mailing_lists_davero_addons_mozilla_org-composeAction-toolbarbutton.toolbarbutton-1.webextension-action
{color:black !important;
background: cadetblue !important; }
/* Verfassen Fenster beide Buttons "Kopie(CC)" "Blindkopie(BCC)" neben "Von" ausblenden */
#addr_ccShowAddressRowButton,
#addr_bccShowAddressRowButton {
display: none !important;}
/* Verfassen Fenster "Kopie(CC)" wieder einblenden */
#addressRowCc.address-row.hidden {display: flex !important; }
/* Verfassen Fenster "Blindkopie(BCC)" wieder einblenden */
#addressRowBcc.address-row.hidden {display: flex !important; }
/* Verfassen Fenster [Kontakt-Seitenleiste] "Kopie(CC)" ausblenden */
#ccButton {display: none !important; }
/* Farbe und Text Dateianhang */
#attachmentBar {
color: black !important; background-color: cadetblue !important; }
#attachmentInfo #attachmentSize {
color: blue !important; font-weight: bold !important; }
/* Zweite Reihe */
.attachmentcell-text .attachmentcell-size {
color: white !important; font-weight: bold !important; }
/* Seitenleiste Aufgaben und Chat Button ausblenden */
#chatButton.spaces-toolbar-button {
display: none !important;}
#tasksButton.spaces-toolbar-button {
display: none !important;}
/* Farbe in der Nachricht Absender / Enpfänger allgemein */
#expandedfromRow .emaillabel {
color: #98F5FF; font-weight: bold !important; }
#expandedtoRow .emaillabel {
color: #FFD39B; font-weight: bold !important; }
#expandedccRow .emaillabel {
color: #FFD39B; font-weight: bold !important; }
#expandedbccRow .emaillabel {
color: yellow; font-weight: bold !important; }
/* Menüleiste "Termine und Aufgaben" ausblenden */
#mail-menubar menu#menu_Event_Task {display: none !important;}
/* Farbe AppMenu */
#button-appmenu {color:#98F5FF !important;}
/* Farbe AddOns-Button */
#button-addons {fill:#66CD00 !important;}
/* Farbe Trash-Button */
#hdrTrashButton {fill: red !important;}
/* New-Tab-Button ausblenden */
#new-tab-button {display:none !important;}
/* Farbe Close Button */
.titlebar-close { background-color: #4C4C50 !important;}
.titlebar-close:hover { background-color: #E81123 !important;}
