I'm experimenting with that right now on firefox with userChrome.css changes, and it's starting to work:

/* Kinda unrelated, but I'm using this to go with Sidebery / Tree Style Tab */

#TabsToolbar { visibility: collapse; }

/* Hide the whole top bar */

#nav-bar {

  position: fixed;
  top: -10vh;
  width: 100%;
  z-index: 1;
  height: 0;
  min-height: 0 !important;

}

#urlbar-container:focus-within, #search-container:focus-within {

  position: fixed;
  top: 15vh;
  left: 50%;
  background: #111;
  outline: 4px solid #f8b218;
  box-sizing: border-box;
  padding: 1em;
  transform: translateX(-50%);
  width: 40vw;
  border-radius: 4px;

}

Top bar stays hidden, but url bar / search bar appear when I press ctrl+l / ctrl+k. Likely need to tweak css to your likings, and you need to set toolkit.legacyUserProfileCustomizations.stylesheets to true in about:config to enable userChrome.css (you can find various guides on that if needed)