This add-on does one thing and it does it well. I wish this were part of the core firefox capability but it is not.

This add-on is a very simple and user-friendly way to get powerful labeling of your windows, so you can go straight to the one you want without having to remember which last-focused tab's title corresponds to the window you want.

Especially useful if you have a lot of windows and tabs and want to group tabs together by related content in certain windows.