This can be changed via chrome/userChrome.css.

Mine is:

  @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
  
  .tabbrowser-tab {
    min-width: 3em !important;
    clip-width: 3em !important;
    
  }
  
  [uidensity="compact"]:root {
    --tab-min-height: 30px !important;
    --newtab-margin: -3px 0 -3px -3px !important;
  }
  
  .tabbrowser-tab {
    max-height: var(--tab-min-height) !important;
  }
  
  .tabs-newtab-button{
    margin: var(--newtab-margin) !important;
  }