Hi, my name is ***** ***** welcome to JustAnswer!
That is not a legitimate Firefox page. Occasionally Firefox may give you a message saying that an update is available, but it doesn't lock your computer up or try to scare you with "URGENT" warnings. Your son is correct that you should not click on that.
If you've been receiving this message for three months then it sounds like your machine has been infected with ad-ware (i.e. malicious software that can cause pop-ups such as this to appear on your screen). I would recommend doing a full virus scan on your machine. One good anti-malware program you can use is called Malwarebytes and you can download the free version from www.malwarebytes.com.
If you ever receive similar messages, or ever receive a pop-up that freezes your screen and gives you a number to call, I can assure you that is not a legitimate message. It is a scam.
If you have any other questions on this, just let me know.
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