If you visit a site that exploits the WebRTC vulnerability including whatismyipaddress.com it may show "Local IP Address" which would be the real IP of your machine.
The vulnerability works on Windows and Linux machines (contrary to false information on the web which claims it only affects Windows).
The above works and happens even if you are using a proxy or VPN account like ProVPNAccounts.com
It is easy to fix the WebRTC IP Address Leak Vulnerability in Firefox and similar browsers in About:Config
Go into about:config and then search for "peerconnection.enabled"
Set it so the value is "false" like the screenshot from Firefox below.