You need to use CSS code in userChrome.css to move the Tab bar below the Navigation Toolbar.

You can look at this thread for CSS code in userChrome.css for Firefox 112+ to move the tabs to below the Navigation Toolbar.

• /questions/1410524 FF 112 Tabs on the bottom beneath the bookmarks toolbar

Hello

Additional information Option of putting the tabs BELOW the address bar https://connect.mozilla.org/t5/ideas/option-of-putting-the-tabs-below-the-address-bar/idi-p/2382 is now open to votes (aka kudos) and comments.