Note that if you get a pop-up message asking to update Firefox or plugins or scanning for malware then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• plugins should only be updated via the plugin itself or by visiting the home page of the plugin.

See this tread in the contributors forum.

• https://support.mozilla.org/forums/contributors/712056

You can use Help > Report deceptive website ... from right inside of Firefox.

That will take you here with the URL already filled in. https://www.google.com/safebrowsing/report_phish/