https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onRemoved
tabs.onRemoved - Mozilla | MDN
Fired when a tab is closed.
tabsmozillamdn
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/windows/onRemoved
windows.onRemoved - Mozilla | MDN
Fired when a window is closed.
windowsmozillamdn