That does not look like a Firefox message.

. Whenever you get a message / popup like that;

DO NOT USE ANY OF THE PROVIDED LINKS OR ANY PHONE NUMBERS ! !

Such messages are only Spam that could lead you to Fake Support. Not only can they damage or hack your system, they charge you to do so.

You can report such a site at; Google Report Phishing Page {web link} which is the same when done while on site by going to Help > Report Web Forgery

Help us safeguard Mozilla’s trademarks by reporting misuse {web link}

How do you mean; 'locked'? Are you saying the page will not go away?

If it's like the fake FBI/Interpol browser lock page, This add-on can stop such pages; https://addons.mozilla.org/en-US/firefox/addon/disallow-script-button/ disallow Script Button

The Disallow Script button looks like a letter "M" and the title is the Minus Script, drag and drop the button on a toolbar. If the button is not displayed then nothing operates, except rules for plugins.

Another way out of the trap 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.