New versions of Firefox removed some support for extensions that allowed multiple tab rows. Here is how you can enable it:

  1. Type about:support in url field of firefox, hit enter.
  2. On the ‘Profile Folder’ setting (about 8th from the top) click the ‘Open Folder’ button.
  3. Create a ‘chrome’ subfolder, if you don’t already have one.
  4. Inside that folder create a file called ‘userChrome.css’ (right click > new > text document – make sure to name the file userChrome.css and not userChrome.css.txt or something)
  5. Open the file for editing and paste the contents of this file;
    https://gist.githubusercontent.com/forexhill/b0446cc31e5001c9e87754df83f0f1ca/raw/f4f1a807d43434a1f10908364dec56ecdf08422c/gistfile1.txt 411
    Into it.
  6. Restart FF and you should have multi row tabs

This comes from user fh1 at this link