Firefox hello is a website that implements webRTC based video conferencing in a browser agnostic way.
The "firefox hello" button that has shown up in recent versions of the browser is a bit of UI magic over an API to this website. The video conferencing code is not implemented in the browser.
If you send a firefox hello link to a chrome user, it opens the webpage when they click on it instead of the UI element.
I actually think it's really neat, and have replaced skype with it since it works so widely and doesn't require everyone to have an account to use it.