WebRTC Permissions UI Toggle 1.0.2
To install the extension, open the file webrtc-permissions-ui-toggle.xpi listed below.
This is an extension for SeaMonkey that adds a toolbar button for toggling the media.navigator.permission.disabled setting on and off. SeaMonkey does not have a permissions dialog to let the user select a camera and microphone (see bug 956854), so overriding the permissions dialog by turning this setting on is the only way to enable WebRTC support.
Of course, leaving this setting on is very insecure, since any web site that's open will be able to get access to your microphone, camera, and screen! So be sure to only turn it on when you need to use WebRTC, and turn it off once you're done with it.
Opening a new browser window will also turn off the permissions dialog override, unless you disable this behavior in the extension preferences.