Some of the security options in Firefox 3 include blocking sites from installing Add-Ons, and storing passwords. You can also assign a Master Password that you must enter in order to "unlock" the rest of your passwords for some added security.
While phishing attacks have been around for some time, attackers continue to innovate and devise new attacks. Recently, one of the emerging methods of spreading malware is by exploiting weak or flawed web servers to plant malware on otherwise benign sites- some very well known. Firefox 3 keeps tabs on the sites that have been identified as compromised, and provides warnings to let you know that you may be visiting a suspected attack site.
Firefox 3 will also let you know if the site you are visiting appears to be a forgery of a legitimate site. This feature is strengthened because rather than just warning you, the contents of suspected forgery sites are not displayed. This makes it substantially more obvious to the user that there may be an issue.