You do not say what the site is... but you the client have communicated successfully with the server, and it has refused you access. It is not your browser which is doing the blocking, it is the server you accessed. Normally, you cannot gain access when you receive such a code, even by password etc.
Not all sites on the web are public.
Looks like you are trying a site where it would require a password from you. I aint very sure about these errors as to what they mean but this one says forbidden which means that you would be able to access. Just so you know websites these days track the IP addresses of the pcs as well to make sure one password is not being used by multiple users and if it happens then they can block that user until the issue is sorted out. That basically would be done by the actual user whom that password belongs to. You are getting it due to the fact that either you are using someone else's password or yours ahve been used by someone else. The latter scenario can be resolved by contacting the admin though.
Usually this error message means that the site administrator of the site you are trying to view has forgotten to make the files public-read again after updating them. I have done that myself a few times.
I have also seen it when a website such as eBay blocked access to an entire ISP because someone was sending spam or malware from there (or faking the ISP return address on it).
I have also seen the error when the site was doing a full backup. Service was restored about 15 minutes later.