Mozilla is keeping its promise of having full support for WebRTC in Firefox by the end of the year (more or less). But it never promised it was going to be enabled by default or that you're going to be able to use it in any meaningful way.
In fact, getUserMedia, one of the first components of WebRTC to get support, is only now being enabled by default and even now it's prefixed.
What it means is that you won't have to fiddle with about:config to turn the feature on, at least not in Firefox 20 Aurora, which has just been released, where it's now enabled by default.
getUserMedia allows web apps to access the webcam and the mic on a computer, with the users' permission. It's crucial for any type of video call technology, which is what WebRTC is, but it's also useful for plenty of other types of apps.
There are plenty of demos that make use of getUserMedia, they should work in Firefox for the most part, though devs have to add Firefox specific code since the feature is prefixed as mozGetUserMedia and that's how it's going to stay until the specs are finalized.