userchrome.css

#navigator-toolbox::after 
{ border-bottom: 35px !important;}

#TabsToolbar 
{ margin-top: 1px !important;
  margin-bottom: 0px !important;
  margin-left: 32vw !important;}

/* Nav bar*/
#nav-bar 
{ background: transparent !important;
  margin-top: -34px !important;
  margin-bottom: -1px !important;
  margin-right: 72vw !important;
  height: 35px !important;
  padding-top: 0px !important;
  font-size:12px !important;}

/* Change Tooltip Appearance */
tooltip {
-moz-appearance:none !important;
background-color: #2b2b2b !important;
color: #e6e6e6 !important;
border-color: #4e4e4e !important;}





.tabbrowser-tab [selected="true"] {
   background-color: #414140 !important;}





.titlebar-spacer[type="pre-tabs"]{
  display: none !important;
}
#navigator-toolbox { --tabs-border-color: transparent !important; }

#urlbar-container {
margin-top: -3px !important;}

#PanelUI-button{ display:block }


#alltabs-button {display: none !important;}

#nav-bar-overflow-button {
display: none !important;}

#urlbar {
margin-top: 2px !important;
length: 33px !important;}













/*Dark context menu*/
menupopup, popup,
popup > menu > menupopup,
menupopup > menu > menupopup {
-moz-appearance: none !important;
border: 1px solid #4e4e4e !important;
padding: 6px 0px 6px 0px  !important;
color: #e6e6e6 !important;
background: #2b2b2b !important;}
menupopup menuseparator {
-moz-appearance: none !important;
margin: 2px 6px 2px 6px !important;
padding: 0 !important;
border-top: none !important;
border-color: transparent !important;
color: #e6e6e6 !important;
background: #2b2b2b !important;}
menupopup menu, menuitem{
-moz-appearance: none !important;
font-size: 9pt !important;
margin: 3px 0px 3px 0px !important;
color: #e6e6e6 !important;
background: #2b2b2b !important;}
menupopup menu:hover, menuitem:hover {
-moz-appearance: none !important;
color:#fff !important;
font-size: 9pt !important;
margin: 3px 0px 3px 0px !important;
background-color:#414141 !important;}

#tracking-protection-icon-container {
    display: none!important;}

/*Faviconize Tabs*/
.tab-label-container { display: none;}
.tabbrowser-tab:not([pinned]) {
max-width: 50px !important;
min-width: 40px !important;}
.close-icon {color: #ffffff !important;}

/* Hide The Sidebar Scrollbar*/
.hidevscroll-scrollbar {display:none !important;}



#PanelUI-menu-button {
list-style-image: url("firefox.png") !important;}
#PanelUI-menu-button > .toolbarbutton-badge-stack {
    padding: 2px !important;}
:root[uidensity="touch"] #PanelUI-menu-button > .toolbarbutton-badge-stack {
    padding: 5px !important;}
#PanelUI-menu-button > .toolbarbutton-badge-stack > .toolbarbutton-icon {
    width: 24px !important;}
#PanelUI-menu-button > .toolbarbutton-badge-stack {
    margin-right: -3px !important;}
#PanelUI-menu-button > .toolbarbutton-badge-stack {
    margin-left: -8px !important;}
#PanelUI-menu-button > .toolbarbutton-badge-stack {
    margin-top: 4px !important;}

#identity-icon {
  display:none !important;}

 
/* ..... Tracking icon ..... */
#tracking-protection-icon-animatable-image {
  display:none !important;}
 
#tracking-protection-icon[state="loaded-tracking-content"] {
 display:none !important;}

image.autoplay-media-icon {
 display: none !important;}

/* Remove empty space before first tab
Delete this rule if you need a space to drag the window*/
#TabsToolbar .titlebar-placeholder[type="pre-tabs"]{
display: none !important;}

/* Remove border between tabs
Delete this rule if you want separators between background tabs */
.tabbrowser-tab::after, .tabbrowser-tab::before {
border-left: none !important;} 

/* Border between background tabs */
.tabbrowser-tab::after, .tabbrowser-tab::before {
display: none !important;}

/* Bookmarks Sidebar */
#bookmarks-view,
#historyTree,
#history-panel,
#sidebar-header {
background-color: #303030 !important;
color: #ccc!important;
border: 0px !important;
border-right: 0px !important;}
#bookmarks-view {
background-color: #303030 !important;
border: 1px !important;
border-right: 0px !important;}
#bookmarks-view {
searchbar-textbox: 
background-color: #303030 !important;
color: #ccc !important;}
#sidebar-header {
display: none !important;
padding-top: 4px !important;
padding-bottom: 0px !important;
background-color: #303030 !important;}
#sidebar-search-container {
background-color: #303030 !important;
padding-left: 0px !important;
padding-right: 0px !important;}
#sidebar-search-container > textbox {
-moz-appearance: none !important;
background-color: #4d4d4d !important;
color: #ffffff !important;
padding-left: 14px !important;
height: 36px !important;}
#bookmarksMenuPopup, #bookmarksMenuPopup menupopup {
-moz-appearance: none !important;
color: #e6e6e6 !important;
background-color: #2b2b2b !important;}
#bookmarksMenuPopup menu, #bookmarksMenuPopup menuitem {
-moz-appearance: none !important;
color: #e6e6e6 !important;
background-color: #2b2b2b !important;}
#bookmarks-menu{
-moz-appearance: none !important;
color: #e6e6e6 !important;
background-color: #2b2b2b !important;}
menupopup {
color: #e6e6e6 !important;
background: #2b2b2b !important;}
:root {
    --arrowpanel-background: #2b2b2b !important;
    --arrowpanel-color: #e6e6e6 !important;
    --panel-background: #2b2b2b !important;}


/*Bookmarks Text Color*/
treechildren::-moz-tree-cell-text(selected),
treechildren::-moz-tree-cell-text(hover),
treechildren::-moz-tree-cell-text(current){
color: #fff !important;
margin-left: 0px !important;}
.sidebar-placesTreechildren::-moz-tree-cell-text(container){
-moz-appearance: none !important;
color: #ccc !important;}

/* Bookmarks sidebar: change hover row color */
treechildren::-moz-tree-row{
background: transparent !important;
height: 20px !important;
border: none !important;
outline: none !important;}
treechildren::-moz-tree-row(hover){
background-color: #1c1c1c !important;}
treechildren::-moz-tree-row(selected){
border: none !important;}
treechildren::-moz-tree-separator{
-moz-appearance: none !important;
border: none !important;
border-bottom: solid 1px rgba(255,255,255,0.4) !important;
background: transparent !important;
min-height: 0px !important;
max-height: 0px !important;}
.sidebar-placesTreechildren::-moz-tree-image(title, container){ 
list-style-image: none !important;
display: none !important;
padding-right: 0px !important;
margin: 0px 0px !important;
width: 0px !important;}

/* Bookmarks Indentation */
#bookmarks-view-children::-moz-tree-indentation {
width: 0px !important;}-indentation {
width: 0px !important;
padding-left: -5px !important;}
#bookmarks-view treechildren::-moz-tree-twisty,
#historyTree treechildren::-moz-tree-twisty {
list-style-image: none !important;
margin-left: -8px !important;}

.sidebar-placesTreechildren::-moz-tree-indentation{ width: 2px !important; }


/* Remove Sidebar border*/
toolbarbutton#alltabs-button {
-moz-binding: url("userChrome.xml");}
#sidebar-splitter{
border: none !important;}

/* Remove Forward Button */
#forward-button {display:none !important;}


/* Move hamburger menu to the left */
#PanelUI-button,
#customization-panel-container,
#customization-panelWrapper .panel-arrow { 
  -moz-box-ordinal-group: 0 !important;
  margin-right: 0px !important;}

/*Hamburger Button Separator*/
#PanelUI-button {border-image-slice: 0 !important;}

/*Sidebar Search Bar*/ 
#sidebar-search-container {display: none !important;}

/* URL Bar */
#pageActionButton,
#pocket-button-box,
ar-button,
.autocomplete-history-dropmarker,
#identity-icon-labels
{display: none !important;}




/* Hide blue stripe on active tab */
.tab-line {
  display: none !important;}


/*Read View Button*/
#reader-mode-button, #pageActionButton {
    display:none!important;}

/* Remove Tab Bar Bottom Line */
:root {--chrome-content-separator-color: none !important;}


.tab-label-container {display: none !important;}
.tab-content {padding: 6px !important;}


/* Show favicon for tabs without favicons */
.tabbrowser-tab:not([pinned]) .tab-icon-image:not([src]) {
display: inline !important;
list-style-image: url('./dot.png') !important;}








/* Show favicon for tabs without favicons */
.tabbrowser-tab:not([pinned]) .tab-icon-image:not([src]) {
display: inline !important;
list-style-image: url('./dot.png') !important;}


/* Remove back button circle */
#back-button:not(:hover),
#back-button:not(:hover) > .toolbarbutton-icon {
background: transparent !important;
border: none !important; box-shadow: none !important; }

#back-button:hover,
#back-button:hover > .toolbarbutton-icon {
border: none !important;
border-radius: 2px !important; }




/* Firefox Context Menu */
#context-navigation,
#context-sep-navigation,
#context-stop,
#context-back,
#context-forward,
#context-reload,
#context-bookmarkpage {
background: #121212 !important;}
#context-stop image:hover,
#context-back image:hover,
#context-forward image:hover,
#context-reload image:hover,
#context-bookmarkpage image:hover {
color: #00000 !important;}
#context-sep-navigation {
display: none !important;}


#tracking-protection-icon-container {
    display: none!important;}

userChrome.js

// ==UserScript==
// @name           FloatingScrollbar.uc.js
// @namespace      [email protected]
// @include        main
// @version        0.0.3
// @note           Thanks to Griever(https://github.com/Griever/userChromeJS/blob/master/SmartScrollbar.uc.js) and Paul Rouget(https://gist.github.com/4003205)
// @note...........0.0.3 Fixed a problem of breaking hbox layout 
// @note           0.0.2 Remove usage of E4X (https://bugzilla.mozilla.org/show_bug.cgi?id=788293)
// ==/UserScript==

(function () {
    var prefs = Services.prefs,
        enabled;
    if (prefs.prefHasUserValue('userChromeJS.floating_scrollbar.enabled')) {
        enabled = prefs.getBoolPref('userChromeJS.floating_scrollbar.enabled')
    } else {
        prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', true);
        enabled = true;
    }

    var css = '\
    @namespace url(http: //www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);\
    :not(select):not(hbox) > scrollbar {\
        -moz-appearance: none!important;\
        position: relative;\
        background-color: transparent;\
        background-image: none;\
        z-index: 2147483647;\
        padding: 2px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "vertical"] {\
        -moz-margin-start: -10px;\
        min-width: 10px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "vertical"] thumb {\
        min-height: 20px;\
    }\
   :not(select):not(hbox) > scrollbar[orient = "horizontal"] {\
        margin-top: -10px;\
        min-height: 10px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "horizontal"] thumb {\
        min-width: 20px;\
    }\
    :not(select):not(hbox) > scrollbar thumb {\
        -moz-appearance: none!important;\
        border-width: 0px!important;\
        border-radius: 0px!important;\
        background-color: rgba(77, 77, 77, 0.9)!important;\
    }\
    :not(select):not(hbox) > scrollbar thumb:active,\
    :not(select):not(hbox) > scrollbar thumb:hover {\
        background-color: #9B9B9B!important;\
    }\
    :not(select):not(hbox) > scrollbar scrollbarbutton, :not(select):not(hbox) > scrollbar gripper {\
        display: none;\
    }';

    var sss = Cc['@mozilla.org/content/style-sheet-service;1'].getService(Ci.nsIStyleSheetService);
    var uri = makeURI('data:text/css;charset=UTF=8,' + encodeURIComponent(css));

    var p = document.getElementById('devToolsSeparator');
    var m = document.createElement('menuitem');
    m.setAttribute('label', "Schwebende Scrollbar");
    m.setAttribute('type', 'checkbox');
    m.setAttribute('autocheck', 'false');
    m.setAttribute('checked', enabled);
    p.parentNode.insertBefore(m, p);
    m.addEventListener('command', command, false);

    if (enabled) {
        sss.loadAndRegisterSheet(uri, sss.AGENT_SHEET);
    }

    function command() {
        if (sss.sheetRegistered(uri, sss.AGENT_SHEET)) {
            prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', false);
            sss.unregisterSheet(uri, sss.AGENT_SHEET);
            m.setAttribute('checked', false);
        } else {
            prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', true);
            sss.loadAndRegisterSheet(uri, sss.AGENT_SHEET);
            m.setAttribute('checked', true);
        }

        let root = document.documentElement;
        let display = root.style.display;
        root.style.display = 'none';
        window.getComputedStyle(root).display; // Flush
        root.style.display = display;
    }

})();

userchrome.xml

<?xml version="1.0"?>

<bindings id="generalBindings"
   xmlns="http://www.mozilla.org/xbl">
  <binding id="js" extends="chrome://global/content/bindings/toolbarbutton.xml#menu">
    <implementation>
        <constructor><![CDATA[
            function makeRelativePathURI(name) {
              let absolutePath = Components.stack.filename;
              return absolutePath.substring(0, absolutePath.lastIndexOf("/") + 1) + name;
            }
            Services.scriptloader.loadSubScript(makeRelativePathURI("userChrome.js"), window);
        ]]></constructor>
    </implementation>
  </binding>
</bindings>