userChrome.css for Firefox with Tree Style Tab. On Windows, put into `%APPDATA%\Mozilla\Firefox\Profiles\XXXXXXXX.default\chrome` directory (create if doesn't exist). Also set toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config.

userChrome.css

/*
 * Edit this file and copy it as userChrome.css into your
 * profile-directory/chrome/
 */

/*
 * Some possible accessibility enhancements:
 *
 * Make all the default font sizes 20 pt:
 *
 * * {
 *   font-size: 20pt !important
 * }
 *
 * Make menu items in particular 15 pt instead of the default size:
 *
 * menupopup > * {
 *   font-size: 15pt !important
 * }
 *
 * Give the Location (URL) Bar a fixed-width font
 *
 * #urlbar {
 *    font-family: monospace !important;
 * }
 */

/*
 * For more examples see http://www.mozilla.org/unix/customizing.html
 */

/*
 * Piotr: customizations for Tree Style Tab addon,
 * see https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#for-userchromecss
 * Update: Firefox 69 or newer requires toolkit.legacyUserProfileCustomizations.stylesheets set to true in about:config.
 */
/* Hide horizontal tabs at the top of the window */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar {
  opacity: 0;
  pointer-events: none;
}
#main-window #TabsToolbar {
    visibility: collapse !important;
}
/* Hide the "Tree Style Tab" header at the top of the sidebar */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
  display: none;
}