From today I cannot open websites starting with letter "b" other websites are running smoothly.Please suggest me what is the possible solution.
I imagine saswata, like most of us, is his/her own admin.
I have no idea how the problem could have occurred. If you cannot get a solution, exit from Firefox, move your ~/.mozilla folder (e.g. by renaming it .mozilla-bad) and re-launch Firefox. You will have lost all your configuration and will have to re-install add-ons. If you have an older version of the .mozilla folder, you could restore that.
Otherwise, ask on the Mozilla forums.