on Mozilla/Firefox/Profiles/xxxxxx/chrome/userChrome.css:

/ Hide Horizontal TAB Bar /
#TabsToolbar {
visibility: collapse !important;
}

/*

• Description: Auto-hide sidebar.
• Contributor(s): img2tab
  */

:root {
--sidebar-min-width: 8px;
--sidebar-visible-width: 200px;
}

#sidebar {
position: relative !important;
min-width: var(--sidebar-min-width) !important;
max-width: var(--sidebar-min-width) !important;
opacity: 0;
}

#sidebar-box:hover :-moz-any(#sidebar,#sidebar-header) {
min-width: var(--sidebar-visible-width) !important;
max-width: var(--sidebar-visible-width) !important;
margin-right: calc((var(--sidebar-visible-width) - var(--sidebar-min-width)) * -1) !important;
z-index:1;
opacity: 1;
}

#sidebar-header,
#sidebar-splitter {
display: none !important;
}