userChrome.css

/* 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/. */

/*
 * Hide tabs on Linux, Windows, any with titlebar.
 * Doesn't work right with MacOS, use the other one instead
 *
 * Contributor(s): Isaac-Newt
 */
/* Hide the whole toolbar area unless urlbar is focused or cursor is over the toolbar */
/* Dimensions on non-Win10 OS probably needs to be adjusted */

/* path  /home/aswin/.mozilla/firefox/v4w2wmp8.default-release/chrome/userChrome.css*/
/*
 *
 * (6) Set Firefox to look for userChrome.css at startup (updated 5/24/2019)

To make startup faster for most users, Firefox 69 will no longer look for this file automatically. You need to tell it to look. Here's how:

    In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
    In the search box above the list, type or paste userprof and pause while the list is filtered. If you do not see anything on the list, please ignore the rest of these instructions. You can close this tab now.
    Double-click the toolkit.legacyUserProfileCustomizations.stylesheets preference to switch the value from false to true.

That change should take effect on your next startup.

***/
@media (-moz-os-version: windows-win10){

  :root[sizemode="maximized"]:not([inDOMFullscreen]){ margin-top: 10px !important; }
  :root[sizemode="maximized"] #navigator-toolbox{ margin-top: -2px }

  @media screen and (min-resolution: 1.25dppx){
    :root[sizemode="maximized"]:not([inDOMFullscreen]){ margin-top: 9px !important; }
  }
  @media screen and (min-resolution: 1.5dppx){
    :root[sizemode="maximized"]:not([inDOMFullscreen]){ margin-top: 8px !important; }
    :root[sizemode="maximized"] #navigator-toolbox{ margin-top: -1px }
  }
  @media screen and (min-resolution: 2dppx){
    :root[sizemode="maximized"]:not([inDOMFullscreen]){ margin-top: 7px !important; }
  }
}

:root[sizemode="fullscreen"]{ margin-top: 0px !important; }

#navigator-toolbox{
  position: fixed !important;
  display: block;
  background-color: var(--lwt-accent-color,black) !important;
  transition: transform 82ms 33ms linear, opacity 82ms 33ms linear !important;
  transform-origin: top;
  line-height: 5px;
  z-index: 1;
}

#navigator-toolbox > *{ line-height: normal }

#navigator-toolbox,
#navigator-toolbox > *{
  width: 100vw;
  -moz-appearance: none !important;
}

#navigator-toolbox:not(:focus-within):not(:hover){
  transform: rotateX(86deg);
  opacity: 0;
}

:root[sizemode="maximized"] #navigator-toolbox:not(:focus-within):not(:hover){ transform: rotateX(89.5deg) }

/* Don't apply transform before window has been fully created */
:root:not([sessionrestored]) #navigator-toolbox{ transform:none !important }

:root[customizing] #navigator-toolbox{
  position: relative !important;
  transform: none !important;
  opacity: 1 !important;
}

#navigator-toolbox[inFullscreen] > #PersonalToolbar,
#PersonalToolbar[collapsed="true"]{ display: none }

/* Uncomment this if tabs toolbar is hidden with hide_tabs_toolbar.css */
 /*#titlebar{ margin-bottom: -9px }*/