Hey, the problem has been resolved
It was not a bug in avast.
I had to re install my firefox browser due to some other problem (I was getting error in downloading youtube videos "No flash video found") and guess what!!! Now the avast web shield is working perfect with firefox too
(please see the attached image)
I never thought it was some problem with firefox or I would have re installed it quite earlier.
But anyways I'm very happy now ... lol