Does exactly what it says on the tin. This is an incredibly basic organization feature that somehow has been missing from life for a long while. Most addons rename tabs or change the naming scheme of the window title, which isn't very useful for per window organization. I separate my tabs per topic in different windows and I go back and forth a lot and having an addon like this helps find things faster.

The only thing I could call a quirk is that if the window's current tab is a new empty tab, the prefix will not appear and the window will simply be "Mozilla Firefox".