Change Firefox UI to i) auto hide address bar; and ii) hide tab bar when there is only one tab opened

userChrome.css

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

/*
 * From https://gist.github.com/BenoitAverty/af633ee20e27f48f9ba7178451432206
 */

#tabbrowser-tabs, #tabbrowser-tabs > .tabbrowser-arrowscrollbox {
	min-height: 0 !important;
}

#tabbrowser-tabs tab[first-visible-tab="true"][last-visible-tab="true"] {
	visibility: collapse;
}

#tabbrowser-tabs .tabs-newtab-button {
	visibility: collapse !important;
}

#tabbrowser-tabs, #tabbrowser-tabs > .tabbrowser-arrowscrollbox main-window[privatebrowsingmode="temporary"]{
	min-height: var(--tab-min-height)
}

#tabbrowser-tabs tab {
	min-height: var(--tab-min-height)
}

/*
 * From https://support.mozilla.org/en-US/questions/1185426
 */

#nav-bar {
  -moz-box-ordinal-group: 1 !important;
  border-top-width: 0 !important;
}

#PersonalToolbar {
  -moz-box-ordinal-group: 2 !important;
}

#TabsToolbar {
  -moz-box-ordinal-group: 3 !important;
}

/*
 * Based on firefox own behavior while in fullscreen
 */

#navigator-toolbox {
	margin-top: -40px;
	transition: margin-top 0.5s;
}

#navigator-toolbox:hover, #navigator-toolbox:focus-within {
	margin-top: 0px;
	transition: margin-top 0.5s;
}

This still works on firefox 92 Stable, seems will still works on 94 nightly. Will test next day. Thanks!

And if we use https://github.com/andreasgrafen/cascade
Then we need only this rules

#navigator-toolbox {
	margin-top: -32px;
	transition: margin-top 0.5s;
}

#navigator-toolbox:hover, #navigator-toolbox:focus-within {
	margin-top: 0px;
	transition: margin-top 0.5s;
}

And it's done. Well this's very powerfull indeed. Than you.

This make browser as native Application!