Firefox Proton (Firefox 89) uses tabs on top
How to place tabs on bottom
Taming the dragon: enabling the CSS
This article shows you how to change Firefox’s appearance from tabs on top (where the tabs appear above the URL address bar) to display the tabs below the URL address bar with CSS and a small browser tweak.{footnote}Before the release of Firefox v57, it was possible to change the location of the tabs by changing a browser parameter setting. FF89 does not have this setting.{/footnote}