Some websites work - others suddenly do not

Certain websites are simply not working. Most websites are fine but my webmail is suddenly not accessible. It works fine on computers elsewhere, for instance my office.

Browsers Firefox and Safari have the same problem.

My version is Norton Security 6.4

I have looked for threads for help but without success. I am guessing it could be the firewall. I cannot find a place where I can list "safe" url's.