After this window has been opened, it will record all payments people send to you (ie, tips), as well as payments you send to others. The tip tracker window is a handy way of keeping track of those avatars with whom you have been financially interacting. You may find it helpful for easily finding the names of people to thank.
The names of financial senders/recipients are clickable, and doing so will take you to that avatar's profile. You also have the option of exporting the data to a CSV file.
If you minimize the window, it will continue to collect entries as they arrive. If you close the window, it will erase its data to free up memory. However, if you enable Track when closed, at the bottom, it will continue tracking even when closed.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.