firefox-csshacks/chrome/urlbar_info_icons_on_hover.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/urlbar_info_icons_on_hover.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Hides info icons (except remote control icon) on the left side of urlbar and shows them when urlbar is hovered. Also hides container name and shows it on hover*/

#urlbar-search-button,
#tracking-protection-icon-container,
#identity-box,
#identity-box.extensionPage #identity-icon-label{
  max-width: 0px;
  transition: max-width 100ms linear !important;
  overflow: hidden;
}

#identity-box.extensionPage{ max-width: 32px }

#urlbar:hover .urlbar-input-container > box,
#urlbar:hover #identity-box.extensionPage #identity-icon-label{
  max-width: 300px;
}
#urlbar:not(:hover) #userContext-label{ display: none }