Firefox Quantum Roomy Bookmarks Toolbar

FF57+RoomyBookmarksToolbar.md

In your Profile folder, add/edit the file chrome/userChrome.css with the following contents:

/* Recreates the basic functionality of the popular Roomy Bookmarks Toolbar add-on:
Hide bookmarks bar items label text, show on hover. */

.bookmark-item > .toolbarbutton-text {
  margin-top: -1px !important;
}
.bookmark-item {
  margin: -1px !important;
}

.bookmark-item:not([container]):not([open="true"]) > .toolbarbutton-text {
  display: none !important;
}

#PlacesToolbarItems > .bookmark-item:not([container]):not([open="true"]) > .toolbarbutton-icon[label]:not([label=""]) {
  margin-inline-end: 0px !important;
}

Profile folder:

Windows: %APPDATA%\Mozilla\Firefox\Profiles\xxxxxxxx.default\
Ubuntu: ~/.mozilla/firefox/xxxxxxxx.default/
Firefox: Menu > Help > Troubleshooting Information > Application Basics > Profile Directory