This seems to encourage using the same credentials everywhere which I think most agree is a "bad idea." If BrowserID is compromised, the attackers have access to all the sites where I use browser id, right?
The same could be said about OpenID and OAuth providers like Twitter, Facebook, Google, etc. The idea of having a centralized authentication provider is definitely not a new one. There are many advantages to using 3rd party authentication providers. The biggest security advantage 3rd party auth providers have is that they do security right and often they implement their authentication service using a commonly defined protocol. Many websites that choose to roll their own systems make many security mistakes and many people use the same credentials on many sites. So if any website is not secure authentication a user's login information is likely compromised across multiple websites because people generally use the same credentials.