Google is able to check the entire web for suspicious and dangerous pages.
Google provides a free service for us to query the status of a page.

Firefox 3 helps keep users safe by using this service to check pages you request.
If Google says a page is dangerous, then it probably is.

If you do not wish to trust Google's service, you may disable this feature in Firefox 3 like so:
Open Tools > Options > Security
Uncheck the option "Tell me if the site I'm visiting is a suspected attack site"

However, I strongly recommend that you keep this option enabled, as a single visit to an attack site
could immediately infect your computer.

If you own the site, I recommend you:
Set up a Google Webmaster Tools account to get more feedback on the problem.
Take down the site to protect your users.
Read what Google has to say about suspicious websites:
http://www.google.com/support/webmasters/bin/answer.py?answer=45432
Read tips on how to clean and secure your website:
http://www.stopbadware.org/home/security