WebRTC Permissions UI Toggle 1.0
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.
Of course, leaving this setting on is very insecure, since any web site will be able to get access to your microphone and camera! So be sure to only turn it on when you need to use WebRTC, and turn it off once you have your webcam up and running.
This extension might also work in Firefox (tested with Firefox 52.0a2), but since it's unsigned, you'll need to use the Developer Edition or a non-branded build. However, since Firefox already has a WebRTC permissions dialog, this extension is not recommended for Firefox and should not be necessary.