I tried this but unfortunately you do not gain any vertical space as the tab bar is still in use as window title bar (or whatever the proper names are for this)

I tried this but unfortunately you do not gain any vertical space as the tab bar is still in use as window title bar (or whatever the proper names are for this). The vertical tab bar is also fixed width (!) and to top it off, after testing nightly for an hour, I closed and reopened the browser and my pinned tabs were gone. I will wait until they figured this out properly. I know it's "nightly" and "labs" so not a complaint per se, but an observation.