Something I wanted in Firefox for years was vertical tabs so that they appear on the left edge of the screen. A few days ago, Firefox updated to 138.0.4, and alas it now has vertical tabs. The problem is though is that I can't make it auto-hide in full screen like I can do with the horizontal tabs. I always work in full screen mode. To make the horizontal tabs appear, I move my mouse to the top of the screen to make them appear so that I can click on another tab, then it disappears again when I move the mouse away from the top. I wanted the same behaviour for the vertical tabs but doesn't seem to be an option at the moment.
about:config
Search for "sidebar.revamp" and change it from true to false.
This disables vertical tabs for the time being until I can find a way of it auto-hiding.
Even better would be an option to have vertical tabs only and no horizontal tabs.