Fullscreen + overlay title bar for Firefox 71+

userChrome.css

/* Fullscreen + overlay title bar for Firefox 71+ */
/* Adapted from https://bugzilla.mozilla.org/show_bug.cgi?id=740148 */

/* Fullscreen support */

/* Shift the browser towards the top */
:root[inFullscreen] {
	--user-fullscreen-delta: calc(-40px - var(--tab-min-height));
}
:root[inFullscreen]:not([inDOMFullscreen]) #browser {
	margin-top: var(--user-fullscreen-delta);
}

:root[inFullscreen] #navigator-toolbox {
	margin-top: var(--user-fullscreen-delta);
	margin-bottom: calc(0px - var(--user-fullscreen-delta));
	z-index: 1;
	background-color: rgba(12, 12, 13, .67);
	backdrop-filter: blur(6px);
}

:root[inFullscreen] .tabbrowser-tab::before,
:root[inFullscreen] .tabbrowser-tab::after {
	border-left-color: rgba(0, 0, 0, .03);
}

:root[inFullscreen] .tabbrowser-tab .tab-background,
:root[inFullscreen] .tabbrowser-tab .tab-background[selected],
:root[inFullscreen] .browser-toolbar {
	background: none transparent !important;
}

:root[sizemode="fullscreen"][inFullscreen] #navigator-toolbox,
:root[sizemode="fullscreen"][inFullscreen] #navigator-toolbox::after {
	transition: all .12s ease .28s !important;
}

:root[inFullscreen] #navigator-toolbox:focus-within,
:root[inFullscreen] #navigator-toolbox:hover,
:root[inFullscreen][debug] #navigator-toolbox {
	transition-delay: 0s !important;
	margin-top: 0;
	margin-bottom: 0;
}