Window Namer by Alvin Ahammed

Window Namer lets you give each Firefox window a short custom label that shows up right in the window title, so when you're Alt-Tabbing or scanning your taskbar with a dozen windows open, you can actually tell which one is "Work," "Personal," "Research," or whatever you call it, instead of guessing from a generic page title.

Click the toolbar icon, type a label for the current window, and save. The label sticks to that window and survives Firefox restarts (as long as Firefox is set to restore your previous session, or you reopen the window via History → Recently Closed Windows).

No tracking, no page content read or modified, no account required — it only touches the window title.