This is a demo that allows websites to detect and connect throwaway accounts to their real users when the user is using Firefox's Private Browsing mode. This exploits Firefox's Bug #1100154 that I reported in 2014. Chrome users are not affected by this exploit.
This exploit could be used by sites that allow users to create throwawy or anonymous accounts (Reddit, HackerNews, 4Chan, etc.) to secretly detect a user's real username if that user is using a Firefox Private Browsing window for the throwaway account while keeping their real account open in their main browser session.
How the exploit works