CSD + Breeze Dark

userChrome.css

/* Adjust Colors */
.tab-background:not([selected=true]),
.tab-background[selected="true"],
#titlebar-buttonbox,
#TabsToolbar,
#PersonalToolbar,
#nav-bar {
	background: #31363b !important;
}

#PanelUI-menu-button {
	margin-right: 6px !important;
}
#back-button {
	margin-left: 6px !important;
}

/* No Black Line */
#navigator-toolbox {
	margin-top: -1px !important;
}

/* Height */
#nav-bar {
	min-height: 40px !important;
	z-index: 3
}
#urlbar {
	border: 1px solid #616569 !important;
	box-shadow: none !important;
	min-height: 30px !important;
	background: #232629 !important;
	color: #eff0f1 !important;
	border-radius: 3px !important;
}

#urlbar:hover {
	border: 1px solid #3daee9 !important;
}

.tabbrowser-tab[fadein]:not([pinned]) {
	max-width: unset !important;
}

/* Adjust Pinned Tab Width */
.tabbrowser-tab[pinned="true"] {
	min-width: 40px;
}
label[pinned="true"] {
	display: none !important;
}
.tab-icon-image[pinned="true"] {
	margin-left: 4px
}

/* Clean Up */
.tab-close-button.tab-close-button,
#page-action-buttons {
	display: none;
}

/* Autohide Tabs */
#TabsToolbar {
	min-height: 40px !important;
	margin-top: -40px !important;
	transition: all 0.3s ease 0s !important;
	-moz-box-ordinal-group: 2 !important;
}
#navigator-toolbox {
	transition: all 0.3s ease 0s !important;
}
#navigator-toolbox:hover {
	margin-bottom: -39px !important;
	transition: all 0.3s ease 0s !important;
}
#navigator-toolbox:hover > #TabsToolbar {
	margin-top: -1px !important;
	transition: all 0.3s ease 0s !important;
}

/* Remove Tab Borders */
.tabbrowser-tab::after, .tabbrowser-tab::before {
	border-left: none !important;
}
:root {
	--tabs-border: transparent !important;
}

/* Tab Line At Bottom */
.tab-line {
	-moz-box-ordinal-group: 2;
	min-height: 3px;
}

/* Tabs don't move when dragging. */
#TabsToolbar[movingtab] {
	padding-bottom: -0px !important;
}

#TabsToolbar[movingtab] > .tabbrowser-tabs {
	padding-bottom: -0px !important;
	margin-bottom: -0px !important;
}

#TabsToolbar[movingtab] + #nav-bar {
	margin-top: -0px !important;
	pointer-events: none;
}

/* Client Side Decorations */
#titlebar-buttonbox {
	height: 40px !important;
}

#nav-bar {
	margin-left: 84px;
	box-shadow: none !important;
}

#titlebar-buttonbox {
	z-index: 3 !important;
}

.titlebar-placeholder {
	display:none !important;
}

#titlebar-max {-moz-box-ordinal-group: 1;}
#titlebar-content {direction: rtl !important;}
#TabsToolbar {direction: rtl !important;}
#tabbrowser-tabs {direction: ltr !important;}

/* Hide window controls when maximized */
:root[sizemode="maximized"] #titlebar-close {
	opacity: 0 !important;
}

:root[sizemode="maximized"] #titlebar-max {
	opacity: 0 !important;
}

:root[sizemode="maximized"] #titlebar-min {
	opacity: 0 !important;
}

:root[sizemode="maximized"] #nav-bar {
	margin-left: 0px;
}