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

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

/* Headerbar */
#nav-bar {
	background: var(--gnome-headerbar-background) !important;
	padding: 6px 3px !important;
}

/* Add private window headerbar indicator */
:root[privatebrowsingmode="temporary"] #nav-bar toolbarspring:first-of-type:before {
	background: url("../icons/eye-not-looking-symbolic.svg") no-repeat;
	background-size: 64px 64px;
	content: "";
	display: block;	
	position: absolute;
	width: 64px;
	height: 46px;
	top: 0;
	transform: translate(15px, 0);
	fill: var(--gnome-toolbar-color) !important;
	fill-opacity: 0.2 !important;
	-moz-context-properties: fill, fill-opacity;
  }

/* Hide tabsbar default private browsing indicator */
#private-browsing-indicator-with-label {
	display: none !important;
}

/* Overlay headerbar on fullscreen */
#navigator-toolbox[inFullscreen] {
  position: fixed !important;
  transition: margin 300ms ease-in-out 0s !important;
  line-height: 0;
  z-index: 1;
  width: 100vw;
}

#navigator-toolbox[inFullscreen]:hover,
#navigator-toolbox[inFullscreen]:focus-within {
  margin-top: 0 !important;
}

#navigator-toolbox[inFullscreen] > * {
  line-height: normal;
}