While Beta 1 of Firefox 2.0 includes built-in anti-phishing tools -- based on an embryonic blacklist -- earlier editions need help from outsiders to warn you of suspicious sites.
The best-known anti-phishing toolbar for Firefox is a free-of-charge download from U.K.-based security vendor Netcraft. The community-supported toolbar -- that means users are the ones who sniff out most of the nasty sites -- blocks suspected URLs, displays a risk ranking for others, and inserts an icon to indicate the site's country of origin.
Other options exist, however, including Google's "Safe Browsing" extension, which adds an icon to the Firefox address bar when you surf to a spoofed site. (Safe Browsing is also part of the Google Toolbar for Firefox; the technology is also the basis for Firefox 2.0's anti-phishing defense.)