Yahoo is banned in firefox - Unable to open yahoo but can open other

I use firefox and Internet explorer but from last few days whenever I try to open yahoo websites I get error saying that Yahoo is banned.
It sometimes open in IE but never in Mozilla firefox.

Plz suggestion solution.

it's an adware use avira premium antivir as a trial and scan your pc.

when you take yahoo.. It is showing some exception .. I guess.. then one option will come
which you may not noticed some security certificate ...
Which is juz like when der is no connection to net... Then you click on that field which comes in the place of try again field then they will ask for adding exception do it Now you can access Yahoo

Jack, its just virus / adware as Kammy suggested. Try some good antivirus and malware remover. Should be solved then.