Install uBlock Origin, avoid third-party Facebook tools entirely, and report any “viewer” websites to Facebook’s security team via their White Hat portal. Your account integrity is worth more than a stranger’s cover photo.
In the past, developers and tech-savvy users found minor loopholes in Facebook’s mobile site architecture to view slightly larger versions of locked photos. However, Facebook regularly patches these vulnerabilities. The Historic "M-Basic" Method fb locked profile cover photo viewer
Are you trying to that was compromised by a scam site? Install uBlock Origin