Firefox userChrome to autohide Sideberry panel and hide titlebar tabs, and autohide Page Actions.

userChrome.css

/*@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");*/
/* ^ disabled namespace, so fullscreen state can be processed */

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*
 * This style will hide the tab bar. For MacOS Big Sur
 *
 * Contributor(s): Isaac-Newt, Ivan0xFF, millerdev, AMomchilov
 */


/* Mac Specific Rules
#titlebar {
  visibility: collapse !important;
}
#TabsToolbar .titlebar-buttonbox-container {
  display: block;
  position: absolute;
  visibility: visible;
}
:root:not([uidensity]) #TabsToolbar .titlebar-buttonbox-container {
  margin-left: 12px;
  margin-top: 12px;
}
:root[uidensity="compact"] #TabsToolbar .titlebar-buttonbox-container {
  margin-left: 10px;
  margin-top: 9px;
}
#TabsToolbar .titlebar-buttonbox.titlebar-color {
  margin-left: 0px !important;
}
#main-window:not([inFullscreen]) #nav-bar {
  padding: 0px 0px 0px 70px !important;
}
*  Mac specific rules */



/* Win7 specific rules */
.tabbrowser-tab {
  visibility: collapse;
}
.titlebar-button {
  height: 27px !important;
}
#nav-bar {
  margin-top: -42px;
  margin-right: 115px;
  box-shadow: none !important;
}
[uidensity="compact"]:root .titlebar-button {
  height: 32px !important;
}
[uidensity="compact"]:root #nav-bar {
  margin-top: -32px;
}
#titlebar-spacer {
  background-color: var(--chrome-secondary-background-color);
}
#titlebar-buttonbox-container {
  background-color: var(--chrome-secondary-background-color);
}
.titlebar-color {
  background-color: var(--toolbar-bgcolor);
}
/* Win7 specific rules */



/*  Sidebery Dynamic sidebar   */
#sidebar-box {
  overflow: hidden;
  height: calc(100% - 72px);
  position: fixed;
  max-width: 120px;
  display: block;
  transition: 90ms;
  z-index: 3;
}

#sidebar-box:hover {
  max-width: 100%;
}

#sidebar {
  width: calc(30px * 10) !important;
  max-width: 50vw !important;
  height: calc(100% - 42px);
}

#appcontent {
  margin-left: 120px;
}

/* Minimize fullscreen transition time */
#navigator-toolbox[fullscreenShouldAnimate] {
  transition: 100ms !important;
}

/* Auto-hide sidebar in fullscreen */
#main-window[inFullscreen="true"] #sidebar,
#main-window[inFullscreen="true"] #sidebar-box,
#main-window[inFullscreen="true"] #sidebar-splitter {
  display: none !important;
  width: 0px !important;
}

#main-window[inFullscreen="true"] #appcontent {
  margin-left: 0px;
}

/* Page Actions Hide and Show on Hover */

/* Hide Buttons and Reveal on Hover */

#page-action-buttons:not(:hover) .urlbar-page-action,
#page-action-buttons:not(:hover) #star-button
{
  width: 0px !important;
  min-width: 0px !important;
  padding-left: 0px !important;
  padding-right: 10px !important;
  margin-right: -5px !important;
  transition: all 250ms ease-in-out;
}

#page-action-buttons:not(:hover) #userContext-indicator {
  margin-right : 37px !important;
  transition: all 250ms ease-in-out;
}

#page-action-buttons:hover .urlbar-page-action,
#page-action-buttons:hover #star-button  {
  visibility: visible;
  min-width: unset !important;
}

#page-action-buttons:hover #userContext-indicator {
  margin-right : 0px !important;
}

/* Create page actions hover "button" */
#page-action-buttons::after {
  content: "•••";
  position: absolute;
  top: 0.7em;
  font-size: 0.7em;
  opacity: 0.5;
  right: 8px;
  transition: all 50ms ease-in-out;
}

/* Hide the button on hover */
#page-action-buttons:hover::after {
  display: none !important;
  width: 0px !important;
  margin-left: 0px !important;
  transition: all 50ms ease-in-out;
}

/* Hide Buttons and Reveal on Hover Finished*/
#urlbar,#searchbar {
  font-size: 13px !important;
	margin-top: 1px !important;
}

Modified for IceCat on Windows 7 with default DPI

• New Tab button at the bottom of Sidebery is now visible;
• Increased collapsed bar width;
• Decreased grip space between Menu button and Window control buttons;

• Disabled sidebar in fullscreen mode
• Decreased fullscreen transition time (for F11 only, for youtube fullscreen transition search for full-screen-api.transition-duration in about:config)

• Disabled namespace, so fullscreen state can be processed