It's something that has become attached to the browser.
These are usually and so far always fakes to get you to go to some site to download a fix or call in and pay up.
If you have saved your bookmarks and cookies (Mozilla and the web have told how many times) just reset the Firefox browser and reload your bookmarks and cookies if you think you need them.
Hello, I keep getting a audible pop-up screen in full red page saying 'Internet Security Alert' and not to shut down my computer. The web address is monitor-inc.com and gives a phone number to call. I took a screen shot and will post if I can. The only way to get rid of this screen is to 'control, alt, delete' to bring up the task master and 'end task' from there. It pops-up every time I open a browser in Firefox. What is this? Is it a true security alert? I have received this alert since I bought the new laptop in November, 2017. How do I get rid of it if fake? Thank you.