Under construction. See: in Mozilla's Bugzilla -- bug 368255, my explanation related with this report. See also page about "advanced antiphishing protection".
Screenshot [1] of window with related default options (See menu Edit -> Preferences... (Linux) or Tools -> Options (Windows), then Security tab.)
More to come... For now you may read this (especially note that browser sends visited URL to Google when browser shows the warning) and this (links found here).
Footnotes:
[1] | In next to last official release in stable line of Firefox 2 (ie. 2.0.0.19) so called "phishing protection" has been disabled by default and the window with related options looks a little bit different than on the screenshot, ie. related options are inactive and the checkbox near "Tell me..." is unchecked. |