userChrome.css

#TabsToolbar {
  visibility: collapse;
}

#webext-panels-browser, #webextpanels-window {
  background: none !important;
}

#sidebar-box:not([hidden="true"]) {
  position: fixed;
  display: block;
  top: 40px;
  left: 0;
  min-width: 36px;
  max-width: 36px;
  height: calc(100vh - 40px);
  z-index: 2;
  transition: none;
  overflow: visible;
  background: transparent !important;
}

#sidebar-box:hover {
  min-width: calc(20vw + 64px);
  max-width: calc(20vw + 64px);
}

#sidebar {
  overflow: hidden;
  min-width: 72px !important;
  max-width: 72px !important;
  position: fixed;
  transition: all 0.2s ease;
  pointer-events: none;
  opacity: 0.5;
  background: -moz-element(#appcontent);
}

#sidebar-box:hover > #sidebar {
  min-width: 20vw !important;
  max-width: 20vw !important;
  pointer-events: all;
  opacity: 1;
}

#sidebar-header,
#sidebar-splitter {
  display: none;
}

#sidebar {
  height: calc(100vh - 40px); 
}

#sidebar-box:not([hidden="true"]) + splitter + #appcontent {
  margin-left: 72px;
}

#main-window[inFullscreen][inDOMFullscreen] #appcontent {
  margin-left: 0;
}

#main-window[inFullscreen] #sidebar {
  height: 100vh;
}