Reports are coming from WebmasterWorld that Google's Referral ads via Google AdSense, are not displaying for many users. Some have said it is isolated to those who use Comcast Cable, while others also say it is a problem on other ISPs.
The same problem here.
In the last 4 days firefox referrals are not showing up.
Here is a sample Google Referral ad, let me know if you do not see it directly below this line:
If you see it, then I guess you do not have that problem. There is no official word from Google on this as of yet.
Forum discussion at WebmasterWorld.