Perfect Forward Secrecy in TLS I read that TLS does PFS using Diffie Hellman. However, DH can be used even without certificates - so how is DHE-RSA better than plain DHE?  

Is DHE a insecure algorithm, that DHE-RSA is needed?
No, DHE is secure and allows to share a common secret between two parties over an insecure channel. But you cannot know, if the one you share the secret with is the one you want (DHE is vulnerable to man in the middle attacks). So DHE-RSA uses DHE to share a common secret and signs the communication with RSA to make sure, that both persons communicate with the right other person.

So in short: DHE does not provide authenticity. DHE-RSA does.