Hi ace2701,
I have dropped MacAfee's SiteAdvisor add-on a long time ago for Fx or Flock, only ran it inside IE7, and their results are not quite reliable because it is not the actual situation on the net that it checks against, but what is in their databases, and this could be quite different from the momentary situation, with the modern ways to hack sites there is no longer any distinction between good sites and evil sites. The malcreants reduced the validity of these URL checkers quite some bit, believe me....
Better is Finjan Secure Browsing 1.314 add=on or the real-time DrWeb's anti-virus link checker enforced through Nightly Tester Tools for Fx 3. You could also use scandoo.com search-engine and add that to you regular search engines in Fx,it does more or less the same as SiteAdvisor would.
polonus