Thank you LinQ and whois.
I tried as Leo suggested but that method did not seem to work probably because I could not do as it was supposed to.
then I tried the ad-on. It seems to work a lot.But just now, similar message appeared with some other ad site listed in the status bar and I had to restart the firefox. Looks like, as LinQ said, there is 'junk in the trunk'. If anybody knows how to remove that junk, please help me.
Thanks again LinQ and whois.