Vertical tabs made a lot of headlines when they hit nightly months ago and they were a bit jank, never quite feeling like they're part of the browser. After updating to 133.0 though, turning on the sidebar revamp (sidebar.revamp) and vertical tabs (sidebar.verticalTabs) in about:config now work so well!
There's no blank space hovering between the address bar and top of the window, tab previews work well on hover. Finally I can ditch Sidebery and it's pulled me back over from flirting with Zen and I'm using this setup full time now <3
Yes, there's a button to expand it and show the tab labels. In the screenshot, it's the button to the right of the refresh button.