If you have peerguardian then that might be the problem because i've also figured out that peerguardian blocks google and yahoo for some reason.
It said that it's a webspider "(http) tcp4 'firefox-bin (110)' (Google Inc:Webspider and Bots)"
if that's the case then just disable peerguardians filter and that should fix it
I've tried this:
cd /Library/Preferences/SystemConfiguration/
sudo mv preferences.plist preferences.plist.sav
and I still get the same prob. No yahoo.com, no google.com or gmail and no youtube....it's my gmail that concerns me. fortunately i have a pc, on which all of these sites still work. in talking w.mac, they say it's a comcast issue. comcast are morons and are sending someone out on tues to look at things here.