userChrome.css

/* Windows users might also want to consider setting
 * browser.tabs.drawInTitlebar to false in about:config
 */

/*
 * requires `toolkit.legacyUserProfileCustomizations.stylesheets=true` in about:config
 */

/*
 * top bar browser tabs
 */
/*
#tabbrowser-tabs {
  visibility: collapse !important;
}
*/

/* from https://framagit.org/ariasuni/tabcenter-reborn/-/wikis/home */
#sidebar-box:not([hidden]) {
  overflow: hidden;
  min-width: 4vw;
  max-width: 4vw;
  position: fixed;
  transition: all 0.2s ease;
  border-right: 1px solid #ddd;
  margin-right: -4vw;
  display: block;
  z-index: 9999999;
}

#sidebar,
#sidebar-box:hover {
  min-width: 20vw !important;
  max-width: 20vw !important;
}

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

/*
 * ADJUST!
 * You need to subtract the height of the panels above sidebar: nav bar,
 * bokmarks bar, sidebar header, etc. -- whichever you have enabled.
 *
 * 42px subtracted below is the height of the nav bar with default theme.
 * I don't have bookmarks bar enabled, and sidebar header is hidden with
 * CSS above, so this is all I need to do.
 *
 * Compact theme nav bar height is 29px btw, if you use that.
 */
#sidebar {
  height: calc(100vh - 53px);
}

#sidebar-box:not([hidden]) ~ #appcontent {
  margin-left: 4vw;
}

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

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