This is great to see. I used this feature when I had Zen browser installed. What this really solves for me is the pain of using two Firefox windows side-by-side with vertical tabs. Every time I move my cursor from the right window to the left, the tab bar opens and stays open. This would solve that problem. It also saves the small cognitive load required to open another tab and move it into split view on whichever OS I happen to be using in that moment