Google Safe Browsing is an extension to Firefox that alerts you if a web page that you visit appears to be asking for your personal or financial information under false pretences. This type of attack, known as phishing or spoofing, is becoming more sophisticated, widespread and dangerous. That's why it's important to browse safely with Google Safe Browsing. By combining advanced algorithms with reports about misleading pages from a number of sources, Safe Browsing is often able to automatically warn you when you encounter a page that's trying to trick you into disclosing personal information.

Source: http://www.google.com/tools/firefox/safebrowsing/index.html