This is a scam and if you click on a scam link or popup if you go to malware laden sites. This is a lesson to stay away from questionable sites. And since no link was provided my guess is you went to a Black sites that have malware and scambots to trick you into getting money for nothing. So stop going to that site or similar sites. Firefox has nothing to do with what those sites peddle to unsuspecting users. And at this point if rebooting isn't working your system is already infected and fix is to take to computer shop to remove this or search online to find someone that has fixed this.

Hi bob279, Sorry to hear about your issue. In Firefox, here is How does built-in Phishing and Malware Protection work?

You might find an Add-on which may help - https://addons.mozilla.org/en-US/firefox/

I suggest contacting Norton about the issue. They may have advice about tweaking settings which will help prevent this from happening again.

Is that message in a Firefox window?

If it's like the fake FBI/Interpol browser lock page, A way out of a trap page is to (force it if needed) Close Firefox. Wait about 20 seconds for Firefox to finish closing. Right-click on the Firefox icon and select Open New Window. +++++++++++++++++++++++++++++++++++++++++ Sometimes this works. If it opens, close it by Menu > File > Exit. This will flush out that web page from the Session Store.