This was the ONLY extension that prevented my WebRTC leak! When turned on, it broke the Google Voice web page - like it is suppose to. When turned off, Google Voice worked.
WebRTC Leak Test ( BrowserLeaks.com ) also showed that the extension was properly blocking my IP from leaking from behind my VPN.
All the other Firefox WebRTC toggle extensions did not work for me. All the other ones leaked like a hole in the roof.