This window shows a list of locations you have visited recently. It is accessed via the top menu, World → Teleport History. It also requires that you enable Preferences → User Interface -> Interface Windows → Use stand-alone window for teleport history.
You can filter the lists by typing a search string in the top space.
At the bottom are icons which allow you to operate on the window.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.