I see a sibling comment posted a link to some tab bar hiding CSS, but having gone though the same "different levels of out of date" problem myself I'll add my own solution that I'm currently using in Firefox 114 on Windows 11. Not perfectly space efficient, but avoids some issues with totally hiding the window titlebar, since I still wanted to keep the minimize/maximize/close controls up there.

Some of this may be platform specific to Windows (can't speak to window management buttons on other platforms), but hopefully it helps if anyone in this thread needs it or lands here later from search results:

  /*  Hide the tabs within TabsToolbar*/
  .toolbar-items {
      display: none;
  }
  
  /* Make the min/max/close buttons align to the right*/
  #TabsToolbar {
      display: flex;
      flex-direction: row-reverse;
  }
  
  /*  Hide the titlebar spacers, which push the buttons away from the corner */
  .titlebar-spacer {
      display: none;
  }
  
  /* Hide the sidebar header */
  #sidebar-header {
      display: none;
  }

Stupid that this sort of fiddling is required when other browsers (like Edge and Brave) are doing native sidebar tabs, but I do like how compact Firefox's can be, plus being a tree instead of a flat list.

Enabling userChrome.css files and finding where to put it is left as an exercise to the reader.