Up to now only Google Chrome has provided official WebRTC support by default within released versions of their Chrome web-browser. There's been preliminary WebRTC support in Firefox for a while but it wasn't enabled by default. Now in the latest nightly releases ahead of Firefox 20, WebRTC is there by default.
More information on WebRTC can be found via its Google Site. There was also a demo made earlier this month. The WebRTC enabling details for Firefox were shared today via a blog post.