. 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}

Someone else just reported the same thing: https://support.mozilla.org/questions/1169677

A lot of times this is a bogus window from one of your browsers, a scam to charge your for tech support services you don't need. They can be spread through ads on popular sites, or links that look legitimate but lead to such fake warnings.

There are a number of different ways that such dialogs could appear in Firefox, including separate pop-up windows (should have the usual buttons in the upper right corner), and script alerts, which may only have an OK button.

With script alerts, pressing the Esc key several times in a row quickly may prevent the script from showing the dialogs over and over and allow you to close the tab normally. Closing normally is useful because then Firefox won't try to restore that tab automatically in your next session.

Some script alerts that re-open will have a checkbox on the bottom left not to let the site show any more dialogs. If you see that, you can check it and then OK the dialog so it doesn't come back. Here's what that looks like from another thread: https://support.cdn.mozilla.net/media/uploads/images/2017-07-18-23-20-59-db997a.png

What's going on in your Firefox at the moment: can you start it up without those dialogs coming back? If necessary, we can describe how to remove/hide your previous session history files so that Firefox can't try to open that page again.