According to the homepage:

Because Hello is built right into Firefox, you can rest easy knowing that your conversations and information will remain private and secure.

I'm not sure exactly why this is though.

Also, out of curiosity more than anything else, what's a decent web-site that implements the same functionality (i.e. set up a VOIP session, let people join by sending a link, ideally anonymously)? I could've sworn AOL had a service like this, but Google results return nothing.