It is extremely cool how it works. Here is a screenshot of a test page Google has setup:
As you can clearly see, Google puts up a very noticable ballon-style tooltip pointing to the address bar, while darkening the rest of the web page. The tooltip clearly and noticably states that this page is most likely a illegal attempt to get personal/financial information from you.
I hope this becomes an integrated feature in not just Firefox, but other major browsers. What a noticable, simple, and effective way to protect users from phishing attacks.
I plan on installing this on my family/relatives computers to protect them. It should be a must use extension for all Firefox users. Check that, ALL internet users.