userChrome.css tab tree with side-bar auto-hidden + hover and side-bar header removed + resize tab

userChrome.css

/* 
MacOS instructions:

- open about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to True
- mkdir ~/Library/Application Support/Firefox/Profiles/<profile>/chrome/
- copy this file to ~/Library/Application Support/Firefox/Profiles/<profile>/chrome/userChrome.css

Options set:

- Using the side-bar with Tree tab mode
- Auto-hide 
- No sidebar-header 
*/

#TabsToolbar, #sidebar-header {
  visibility: collapse !important;
}

#TabsToolbar {
  margin-bottom: -21px !important;
}

.closebox {
  display: none
}

.tab:hover .closebox {
  display: block;
}

.tab {
  padding-top: 0px;
  padding-bottom: 0px;
}

.tab .icon {
 padding-top: 2px; /* reducing it by 2px */
 padding-bottom: 2px;
}

:root {
  --tab-height: 20px; /* reducing it by 4px */
  --sidebar-hover-width: 20px;
  --sidebar-visible-width: 260px;
}

/* move nav panel in slightly to prevent covering macOS window
controls */

#nav-bar-customization-target {
     padding-left: 60px;
}


#sidebar-box {
  position: relative !important;
  overflow-x: hidden !important;
  margin-right: calc(var(--sidebar-hover-width) * -1) !important;
  left: var(--sidebar-hover-width) !important;
  min-width: var(--sidebar-hover-width) !important;
  max-width: var(--sidebar-hover-width) !important;
}

#sidebar-box:hover {
  margin-right: calc(var(--sidebar-visible-width) * -1) !important;
  left: var(--sidebar-visible-width) !important;
  min-width: var(--sidebar-visible-width) !important;
  max-width: var(--sidebar-visible-width) !important;
}

#sidebar {
  opacity: 0 !important;
}

#sidebar:hover {
  opacity: 1 !important;
}

/* #sidebar-header is hidden by default, change "none" to "inherit" to restore it. */
#sidebar-header {
  display: inherit !important;
}

/* #sidebar-splitter styles the divider between the sidebar and the rest of the browser. */
#sidebar-splitter {
}