I just tried jssip in Chrome with Asterisk (directly and via Kamailio): signaling work (no intensive testing) but audio does not work due to a bug in Chrome. I also tried Opera 12.11 and Firefox nightly 2012-11-27 but it sems that both do not support webrtc at all. Seems like we can only test Chrome vs. Chrome.
WebRTC is in Firefox nightly. I haven't tried it myself because the APIs have slightly different names (start Moz instead of Webkit). This means that WebRTC software needs to detect the browser type and call differently named functions.
Peter