1. Visit about:config
  2. Search and enable toolkit.legacyUserProfileCustomizations.stylesheets
  3. Visit about:support
  4. Click on Open Directory which is to the right of Profile Directory
  5. Create a new folder called chrome
  6. Open chrome and create a new file named userChome.css
  7. Paste the following
#tabbrowser-tabs {
  visibility: collapse;

#titlebar #TabsToolbar {
  background-color: var(--toolbar-bgcolor);
  background-image: var(--toolbar-bgimage);
  padding-top: 5px;

Tested on Firefox v100.0.2.