firefox-gnome-theme/theme/parts/tabsbar.css

/* Tabs bar  */

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

:root {
	--space-above-tabbar: 0 !important;
}

#TabsToolbar {
    --toolbarbutton-inner-padding: var(--toolbarbutton-inner-padding) !important;
	background-color: var(--gnome-tabbar-background) !important;
	border-bottom: 0 !important;
	padding: 0 !important;
	position: relative;
	z-index: 1;
}

#TabsToolbar .titlebar-buttonbox-container:not(#hack) {
	display: none !important;
}

#TabsToolbar:-moz-window-inactive {
	background-color: var(--gnome-inactive-tabbar-background) !important;
}

/* Tabs bar height */
#tabbrowser-tabs,
#tabbrowser-tabs arrowscrollbox {
	height: auto !important;
	min-height: auto !important;
	--tab-min-height: 34px !important;
}

tab > stack {
	height: 34px !important;
	min-height: 34px !important;
}

/* Tabs scroll fade */
#TabsToolbar {
	--gnome-tabbar-fade-background: var(--gnome-tabbar-background);
}
#TabsToolbar:-moz-window-inactive {
	--gnome-tabbar-fade-background: var(--gnome-inactive-tabbar-background);
}

#scrollbutton-up:not([disabled])::after,
#scrollbutton-down:not([disabled])::after {
	content: "";
	height: 100%;
	position: absolute;
	top: 0;
	z-index: -1;
	width: 50px;
}

#scrollbutton-up:not([disabled])::after {
	background: linear-gradient(to right, var(--gnome-tabbar-fade-background) 25px, transparent);
	
}
#scrollbutton-down:not([disabled])::after {
	background: linear-gradient(to left, var(--gnome-tabbar-fade-background) 25px, transparent);
	right: 0;
}

spacer[part=overflow-start-indicator], spacer[part=overflow-end-indicator] {
	width: 0 !important;
	border: 0 !important;
	margin-inline: 0 !important;
}

#scrollbutton-up, #scrollbutton-down {
	position: relative !important;
	z-index: 5 !important;
}

/* Remove hover effects on tab bar buttons */
#TabsToolbar {
	--toolbarbutton-active-background: transparent !important;
	--toolbarbutton-hover-background: transparent !important;
}

/* Tabsbar buttons */
#TabsToolbar .toolbarbutton-1:not(#hack) {
	border-radius: 6px !important;
	margin: 5px 3px 6px !important;
	padding: 0 9px !important;
}
#TabsToolbar .toolbarbutton-1:not([disabled]):not(:active):not([open]):hover {
	background-color: var(--gnome-tabbar-tab-hover-background) !important;
}
#TabsToolbar .toolbarbutton-1:active, #TabsToolbar .toolbarbutton-1[open] {
	background-color: var(--gnome-tabbar-tab-active-background) !important;
}

/* Fake tabsbar border */
#tabbrowser-arrowscrollbox::after {
	content: "";
	display: block;
	width: 200%;
	background: var(--gnome-toolbar-background);
	border-bottom: 1px solid var(--gnome-toolbar-border-color);
	height: 0;
	min-height: 0;
	position: absolute;
	bottom: 0;
	left: 0;
	z-index: 6;
}

#tabbrowser-tabs[movingtab=true] #tabbrowser-arrowscrollbox::after  {
	bottom: unset;
	top: 38px;
}

/* Tab */
.tabbrowser-tab {
	border: 0 !important;
	padding: 5px 3px 6px !important;
	position: relative;
}
.tabbrowser-tab[first-visible-tab="true"]{
	padding-left: 5px !important;
}
.tabbrowser-tab[last-visible-tab="true"] {
	padding-right: 5px !important;
}

#tabbrowser-tabs[haspinnedtabs]:not([positionpinnedtabs]) > #tabbrowser-arrowscrollbox > .tabbrowser-tab[first-visible-unpinned-tab] {
  margin-inline-start: 0 !important;
}

/* Tab separators */
.tabbrowser-tab:not([selected], :hover, [last-visible-tab="true"], [beforeselected-visible="true"], [beforehovered="true"])::after {
	content: "";
	display: block;
	border-left: 1px solid var(--gnome-tabbar-tab-separator-color) !important;
	height: 26px;
	position: absolute;
	top: 9px;
}
.tabbrowser-tab::after {
	right: -.5px;
}

/* Tab labels */
tab {
	color: var(--gnome-toolbar-color) !important;
	font-family: Cantarell, inherit;
	font-weight: normal;
	font-size: 1em;
}

/* Reset tab background */
.tab-background {
	border-radius: 6px !important;
	box-shadow: none !important;
	margin-block: 0 !important;
	border: 0 !important;
}

/* Center all inside tab */
.tab-content {
	display: flex;
	justify-content: center;
	align-items: center;
	margin-top: -1px;
	min-width: 100% !important;
	padding: 0 10px !important;
}

/* Fix custom info tab icon */
.tabbrowser-tab[image="chrome://global/skin/icons/info.svg"]:not([pinned]):not([busy]):not([progress]) .tab-icon-stack::before {
	margin-inline-end: 5.5px;
}
.tabbrowser-tab[image="chrome://global/skin/icons/info.svg"] .tab-icon-image {
	display: none !important;
}

/* Prevent tab icons size breaking */
.tab-icon-image, .tab-icon-sound, .tab-throbber, .tab-throbber-fallback, .tab-close-button {
	min-width: 16px;
}

/* Center tab icon contents */
.tabbrowser-tab .tab-icon-stack {
	align-items: center;
	justify-items: center;
}

/* Adjust tab label width */
.tab-label-container {
	min-width: 0 !important;
}

/* Put tab close button to the right */
.tabbrowser-tab .tab-close-button {
	margin-left: auto !important;
}

/* Force tab favicon to the center */
.tabbrowser-tab .tab-icon-stack {
	margin-left: auto !important;
}

/* Tab icon margin */
.tabbrowser-tab:not([pinned]) .tab-icon-stack {
	margin-right: 2px;
}
.tab-throbber:not([pinned]), .tab-icon-pending:not([pinned]), .tab-icon-image:not([pinned]), .tab-sharing-icon-overlay:not([pinned]), .tab-icon-overlay:not([pinned]) {
  margin-inline-end: 0 !important;
}
.tabbrowser-tab:not([soundplaying], [muted], [activemedia-blocked], [crashed]) .tab-icon-stack {
	padding: 4px;
}

/* Hide favicon when mute icon is present */
.tabbrowser-tab:not([pinned]):is([soundplaying], [muted], [activemedia-blocked], [crashed]) .tab-icon-image:not([sharing]),
.tabbrowser-tab:not([pinned]):is([soundplaying], [muted], [activemedia-blocked], [crashed])[selected] .tab-icon-image {
	display: none;
}

/* Hide secondary label about muting */
.tabbrowser-tab:is([soundplaying], [muted], [activemedia-blocked], [crashed]) .tab-secondary-label {
	display: none;
}

/* Tab buttons */
.tab-icon-overlay,
.tab-close-button {
	border: 0px solid transparent;
	box-sizing: content-box; /* Avoid deformation on flexbox */
	border-radius: 99px !important;
	padding: 4px !important;
	background-size: 24px;
	transition: all 0.2s ease-out !important;
}
:root:not(:-moz-window-inactive) .tab-icon-overlay:hover,
:root:not(:-moz-window-inactive) .tab-close-button:hover {
	background: var(--gnome-button-flat-hover-background) !important;
}

.tab-icon-stack:is([soundplaying], [muted], [activemedia-blocked]) image {
	 margin: auto !important;
}

/* Always show the muted icon when poresent */
#tabbrowser-tabs:not([secondarytext-unsupported]) .tabbrowser-tab:not(:hover) .tab-icon-overlay[indicator-replaces-favicon] {
	opacity: 1 !important;
}

/* Icon overlay smaller */
.tab-icon-overlay[pinned], .tab-icon-overlay:not([indicator-replaces-favicon="true"], [selected]) {
	width: 12px !important;
	height: 12px !important;
	padding: 2px !important;
}
/* Icon overlay small style */
.tab-icon-overlay:is([pinned], [sharing]):not([crashed]) {
	stroke: unset !important;
}
.tab-icon-overlay:is([pinned]):not([crashed]),
.tab-icon-overlay:is([sharing]):not([crashed], [selected]) {
	background: var(--gnome-tab-button-background) !important;
}
.tab-icon-overlay:is([pinned], [sharing]):not([crashed]):hover {
	background: var(--gnome-tab-button-hover-background) !important;
}

/* Fix icon overlay posisition when sharing */
.tab-icon-overlay:not([crashed], [pinned]):is([sharing])[selected] {
	top: 0 !important;
	inset-inline-end: 0 !important;
}

/* Close tab button */
:root {
	--gnome-fill-icon: red !important;
}
.tab-close-button {
	list-style-image: url("../icons/window-close-symbolic.svg") !important;
	fill: var(--gnome-toolbar-color) !important;
	fill-opacity: 1 !important;
	-moz-context-properties: fill, fill-opacity !important;
	height: 16px !important;
	opacity: 1 !important;
	width: 16px !important;
	z-index: 100 !important;
}
:root:-moz-window-inactive .tab-close-button:not(#hack) {
	opacity: .18 !important;
}
:root:not(:-moz-window-inactive) .tab-close-button:active {
	background-image: var(--gnome-button-active-background) !important;
	box-shadow: var(--gnome-button-active-box-shadow) !important;
	border-color: var(--gnome-button-active-border-color) !important;
	border-bottom-color: var(--gnome-button-active-border-color) !important;
	border-top-color: var(--gnome-button-active-border-accent-color) !important;
}

.tab-close-button:active:not(:hover) {
	background-image: none !important;
	box-shadow: none !important;
}

/* Autohide */
.tabbrowser-tab:not([selected="true"]) .tab-close-button {
	visibility: hidden !important;
	opacity: 0 !important;
}
.tabbrowser-tab:hover .tab-close-button {
	visibility: visible !important;
	opacity: 1 !important;
}

/* Fix close button position */
.tabbrowser-tab:not([pinned]) .tab-label-container {
	margin-right: -16px;
}
.tabbrowser-tab:not([pinned]):is([selected="true"], :hover) .tab-label-container[textoverflow="true"] {
	margin-right: 0;
}
#tabbrowser-tabs[closebuttons="activetab"] .tabbrowser-tab:not([pinned]) .tab-close-button {
	display: block !important;
}

/* Close button overlay */
.tabbrowser-tab:not([pinned]) .tab-content::before {
	border-radius: 0 6px 6px 0;
	content: "";
	height: 100%;
	opacity: 0;
	position: absolute;
	right: 0;
	top: 0;
	transition: all 0.2s ease-out;
	width: 55px;
}
.tabbrowser-tab:not([pinned]) .tab-content::before {
	background: linear-gradient(to left, var(--gnome-tabbar-tab-close-overlay-bg) 50%, transparent);
	z-index: 99;
}
.tabbrowser-tab:not([pinned], :-moz-window-inactive):is([selected="true"], :hover) .tab-content::before {
	opacity: 1;
}
.tabbrowser-tab:not([pinned])[selected="true"] .tab-content::before {
	--gnome-tabbar-tab-close-overlay-bg: var(--gnome-tabbar-tab-active-background);
}
:root:not(:-moz-window-inactive) .tabbrowser-tab:not([pinned])[selected="true"]:hover .tab-content::before {
	--gnome-tabbar-tab-close-overlay-bg: var(--gnome-tabbar-tab-active-hover-background);
}
.tabbrowser-tab:not([pinned]):hover .tab-content::before {
	--gnome-tabbar-tab-close-overlay-bg: var(--gnome-tabbar-tab-hover-background);
}
.tabbrowser-tab:not([pinned])[selected="true"]:-moz-window-inactive .tab-content::before {
	--gnome-tabbar-tab-close-overlay-bg: var(--gnome-inactive-tabbar-tab-active-background);
}

/* Tab close button etc. positioning */
.tab-throbber, .tab-icon-image, .tab-sharing-icon-overlay, .tab-icon-sound, .tab-close-button {
	margin-top: 0 !important;
}

/* Active tab */
.tab-background[selected=true] {
	background: none !important;
	border-image: none !important;
}
.tab-background[selected=true]  {
	background-color: var(--gnome-tabbar-tab-active-background) !important;
}
.tab-background[selected=true]:-moz-window-inactive  {
	background-color: var(--gnome-inactive-tabbar-tab-active-background) !important;
}

/* Tab hover */
.tabbrowser-tab:hover > .tab-stack > .tab-background[selected=true] {
	background-color: var(--gnome-tabbar-tab-active-hover-background) !important;
}
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]),
#TabsToolbar[brighttext] > #tabbrowser-tabs > .tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]),
#TabsToolbar[brighttext] > #tabbrowser-tabs > .tabbrowser-tab:hover > .tab-stack > .tab-background > .tab-line:not([selected=true]) {
	background-color: transparent !important;
}
.tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]) {
	background-color: var(--gnome-tabbar-tab-hover-background) !important;
	border-image: none !important;
}
:root:-moz-window-inactive .tabbrowser-tab:hover > .tab-stack > .tab-background:not([selected=true]) {
	background-color: var(--gnome-inactive-tabbar-tab-hover-background) !important;
}

/* Full width tabs */
.tabbrowser-tab:not([style^="max-width"]):not([pinned]),
.tabbrowser-tab[style^="max-width: 100px !important;"]:not([pinned]) {
	max-width: 100% !important;
	min-width: 150px !important;
}
.tabbrowser-tab:not([style^="max-width"]):not([pinned]):not([fadein]),
.tabbrowser-tab[style^="max-width: 100px !important;"]:not([pinned]):not([fadein]) {
	max-width: .1px !important;
}

/* Remove blank spaces on tabs start and end */
#TabsToolbar .titlebar-spacer {
	display: none !important;
}

/* Pinned tab size */
.tabbrowser-tab[pinned] {
	max-width: 37px !important;
}

/* Remove container bottom line indicator */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
	display: none;
}

/* Create new container tab indicator */
.tabbrowser-tab[class*="identity-color-"] .tab-label-container {
	color: var(--identity-tab-color) !important;
}

/* Tab spinner */
.tab-throbber::before {
	animation: gnome-spinner 1s linear infinite !important;
	background-image: url("../icons/process-working-symbolic.svg") !important;
	width: 16px !important;
	opacity: 1 !important;
}
@keyframes gnome-spinner {
	from {
		transform: rotate(0deg);
	}
	to {
		transform: rotate(360deg);
	}
}

/* OPTIONAL: Hide single tab */
@supports -moz-bool-pref("gnomeTheme.hideSingleTab") {
	#tabbrowser-tabs tab:only-of-type {
		display: none !important;
	}
}

/* OPTIONAL: Use normal width tabs */
@supports -moz-bool-pref("gnomeTheme.normalWidthTabs") {
	.tabbrowser-tab:not([style^="max-width"]):not([pinned]),
	.tabbrowser-tab[style^="max-width: 100px !important;"]:not([pinned]) {
		max-width: 225px !important;
	}
}

/* OPTIONAL: Add more contrast to the active tab */
@supports -moz-bool-pref("gnomeTheme.activeTabContrast") {
	.tab-background[selected=true],
	:root:not(:-moz-window-inactive) .tabbrowser-tab:hover > .tab-stack > .tab-background:not(#hack) {
		background: var(--gnome-tabbar-tab-active-background-contrast) !important;
	}
}

/* OPTIONAL: Show the close button on the selected tab only */
@supports -moz-bool-pref("gnomeTheme.closeOnlySelectedTabs") {
	/* Hide the close buttons on hover */
	.tabbrowser-tab:not([selected="true"]):hover .tab-close-button {
		visibility: hidden !important;
		opacity: 0 !important;
	}
	/* Hide the close button overlay on hover */
	.tabbrowser-tab:not([pinned]):hover .tab-content::before {
		--gnome-tabbar-tab-close-overlay-bg: unset;
	}
	/* Reset the overflow position of the tab label container */
	.tabbrowser-tab:not([pinned]):not([selected="true"]):hover .tab-label-container[textoverflow="true"] {
		margin-right: -16px;
	}
}