With horizontal tabs, pressing the F11 key displays the entire screen, without the tabs. They appear as a slide when you move the cursor to the top of the browser. With vertical tabs, the same behavior would be required, but with Firefox's left margi...
No, I speak about "Vertical Tabs", the extra module in Firefox https://addons.mozilla.org/fr/firefox/addon/vertical-tabs-reloaded/Not the feature that is in development in Nightly.