That is distressing. Signing in to a web session or service like Firefox Monitor would not allow someone to grab your Sync data. To see what kind of connection it was, and to disconnect it, see: https://accounts.firefox.com/settings#connected-services If there isn't a device (computer or mobile) next to the connection, it *probably* did not result in a leak of data but unfortunately, I don't know how to be sure of that.