Mozilla is working on implementing full WebRTC support in Firefox. It promised to have it ready in Firefox and things seem to be going according to plan.
Firefox 18 is in Aurora now and it does indeed boast preview support for WebRTC. Firefox has supported getUserMedia API, which handles the webcam and mic − a crucial component of WebRTC, for several months.
But the latest Firefox has an implementation of PeerConnection, the other big component of WebRTC.
The latest Chrome 23 Beta also added support for PeerConnection, so it looks like the end of the year is going to be big for WebRTC.
With the latest additions to Firefox 18 and continuing improvements landing in the Nightly channel, which currently hosts Firefox 19, WebRTC support is shaping up.
That said, the feature is still not enabled by default in Firefox, specifically PeerConnection is behind a config option. You have to navigate to about:config and set media.peerconnection to "true" for it to work.
Granted, there's not much you can actually do with it after that, Mozilla does have test page for WebRTC setup, you can play around there for a bit, but it's going to be a few more weeks or months before you can do calls.