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 also:

• https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update

Hi,

Did this 'warning' tell you to call a phone number for support  ? In that case : needless to say not to call that number.

Did they ask you for authentication  ? In that case :

When you get a scam page that shows that two-line Authentication Required dialog, and you cancel it, usually the page will reload and show it again. To break the cycle, tap the Esc key several times in a row quickly. That should cancel the dialog AND the reload. Then you can easily close the tab.

Also see : https://malwaretips.com/blogs/warning-your-computer-may-be-infected-removal/

Maybe you'd consider installing this add-on, that might prevent this in the future : https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/