Firefox uses Google Safe Browsing to warn users of possible phishing and fraudulent sites. See: http://www.mozilla.com/en-US/firefox/phishing-protection/ To report a suspected site to Google Safe Browsing, click on Help > Report Web Forgery. If you do this while on the tab that you suspect, that tab will be replaced by the Google reporting site, and the URL of the site you just came from will be completed for you; otherwise, you must provide the URL yourself.