Ich benutze im wesentlichen den Code aus Post #1, aber ich hänge mal sicherheitshalber meine komplette userChrome.css an, sollte auch nicht so riesig sein.
CSS
#urlbar[breakout][breakout-extend] {
top: calc((var(--urlbar-toolbar-height) - var(--urlbar-height)) / 2) !important;
left: 0 !important;
width: 100% !important;
}
#urlbar[breakout][breakout-extend] > #urlbar-input-container {
height: var(--urlbar-height) !important;
padding-block: 0 !important;
padding-inline: 0 !important;
}
#urlbar[breakout][breakout-extend][breakout-extend-animate] > #urlbar-background {
animation-name: none !important;
}
#urlbar[breakout][breakout-extend] > #urlbar-background {
box-shadow: none !important;
}
#TabsToolbar {
position: absolute;
display: block;
bottom: 0;
width: 100vw;
background-clip: padding-box;
color: var(--toolbar-color);
}
#tabbrowser-tabs {
width: 95vw;
}
:root {
--tab-min-height: 25px !important;
}
:root #tabbrowser-tabs {
--tab-min-height: 25px !important;
}
#navigator-toolbox {
position: relative;
padding-bottom: calc( var(--tab-min-height) + 8px );
}
.titlebar-buttonbox-container {
position: fixed;
right: 0;
visibility: visible;
display: block;
}
#TabsToolbar .titlebar-buttonbox-container,
#TabsToolbar #window-controls {
display: none;
}
#nav-bar toolbarspring {
background: transparent !important;
max-width: 20px !important;
min-width: 20px !important;
width: 20px !important;
margin-top: 4px !important;
margin-bottom: 4px !important;
}
.bookmark-item[container="true"] {
fill:#D08000 !important;
/* color:blue !important; /* */
}
/*Abstände der Einträge im Kontext wieder schmaler*/
menupopup > menuitem,
menupopup > menu {
padding-block: 0 !important;
padding-inline-start: 4px !important;
}
#PlacesToolbar menuitem,
#PlacesToolbar menu
{
margin: 1 0 0 0 !important;
min-height: 18px !important;
padding: 0 !important;
}
#appMenu-multiView .subviewbutton {
padding-block: 0 !important;
}
#search-container, #urlbar-container {
--urlbar-container-height: 32px !important;
padding-block: 2px !important;
}
#urlbar, #searchbar {
--urlbar-height: 26px !important;
--urlbar-min-height: 26px !important;
--urlbar-toolbar-height: 32px !important;
}
#urlbar-input-container {
--urlbar-icon-padding: 4px !important;
border: 0 !important;
}
/*Das lila Icon für privaten Modus oben rechts eingefügt*/
#main-window[privatebrowsingmode=temporary] #PanelUI-menu-button::before {
margin-left: 0px!important;
margin-right: 0px !important;
content: '' !important;
display: block !important;
width: 19px !important;
height: 19px !important;
background: url("chrome://global/skin/icons/indicator-private-browsing.svg") !important;
background-repeat: no-repeat !important;
background-position: 0px 2px !important;
background-size: 15px 15px !important;
}
/*Der neue zusätzliche Eintrag: Privater Modus unter dem Tab entfernt*/
#private-browsing-indicator-with-label {
display: none !important;
}Danke für das Interesse! 
