Release v137, Win10, vertical tabs, tabs set to the right.
As an experiment to see how small they could get, I resized the new tab bar to the screen edge, and the bar disappeared. Hovering back over, I get a glowing blue bar that tells me I have a drag surface, awesome, but as soon as I start dragging the cursor reverts to a regular pointer icon and nothing happens. I've tried both maximized and in a regular window.
I currently can't figure out how to get them back, but I'm assuming some user css trickery, since it's not in about:config at all. This seems like a pretty important bug, if it's replicable.
Edit: I added the sidebar button back onto the toolbar and clicking that at least reopened it.
