I generally have a "home" Firefox window with my most used tabs pinned. Sometimes I close it before another window, so I was frustrated to "lose" it and having to redo my pins. But recently I discovered this feature. Joy!
Go to the hamburger menu where your settings are and look at your bookmarks and history. It's a way of tracking. Obviously it's not a huge deal or I would use another browser but I still think it's a little shady. It doesn't delete if you close out your browser and have delete history turned on, you can't manually delete it and there's no setting to turn it off. It's obvious you don't mind tracking so you do you.