screen locked on iPhone 7+ Asking to press button OK
on my iPhone 7+ the firefox screen is locked. message says: https://networkdefense-guardian.com Your device is infected and vulnerable to cyber-attacks and data theft! To protect your iPhone press the button below. there are two buttons: CONFIRM is not active OK in colour blue, I assume is active. I have not pressed the button.
How do I prevent this hack, assuming it is a hacker? How do I unlock screen safely so can use firefox?