A large number of malicious websites are using social engineering techniques to display fake security alerts that resemble the legitimate ones the web browsers Firefox and Chrome display when a user navigates to a page that is suspected to be malicious.
The typical social engineering tactic of displaying a false warning message or security alert is nothing new to us because over the years we have witnessed literally hundreds of rogue applications that use this technique.
http://www.enigmasoftware.com/fake-firefox-google-browser-alerts-spread-bogus-security-tool/