https://support.mozilla.org/en-US/questions/925973
type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the filter box, type or paste tabs and pause while the list is filtered
(3) [At end] Double-click browser.tabs.onTop to switch it from true to false to move the tabs below the toolbars.
That method was true a dozen years ago for Firefox 12. They eliminated that easy method to change tab positions when they updated their entire browser system (more than once) and now they are on version 137.