Yes, Mozilla Firefox users can actually turn off the default WebRTC functionality directly in Firefox settings by typing ‘about:config’ into the search bar and browsing to the ‘media.peerconnection.enabled’ option and setting it to FALSE. I have done this in my Firefox.
With my VPN connected I cut and pasted your link ( https://diafygi.github.io/webrtc-ips/ ) in Firefox, Internet Explorer, Opera and Chrome. Only Chrome displayed any IP Addresses (2 different IP's for Chrome) all the others showed blank spaces without any IP's listed at all.
On my VPN forum I found this statement - "This doesn't affect OSX or Android users, seemingly just Windows users." Referring to WebRTC leaks. I am not sure about that statement.