Prevent WebRTC from leaking local IP address
Keep in mind that this feature is to prevent leakage of your non-internet-facing IP adresses. The purpose of this feature is not to hide your current internet-facing IP address -- so be cautious to not misinterpret the results of some WebRTC-local-IP-address-leakage tests found online.
For example, if you use a VPN, your internet-facing IP address is that of the VPN, so your ISP-provided IP address should not be visible to outside world with this setting checked. However, if you are not behind any VPN or proxy, your ISP-provided IP address will be visible regardless of this setting.
Test: Trickle ICE (click the Gather candidates button at the bottom).
If you are using an extension-based VPN, this setting won't prevent your ISP IP address from leaking.
Also: "When using a proxy, WebRTC leaks unproxied IP address even with multiple routes disabled" (reportedly fixed in Chromium 47, see comment #25).
The feature works only on version 42 and above.
With Firefox 41 and lower OR uBlock Origin 1.3.3 and lower, it is NOT possible to prevent local IP addresses leakage without completely disabling WebRTC.
With Firefox 42 and higher AND uBlock Origin 1.3.4 and higher, it is possible to prevent local IP addresses leakage without completely disabling WebRTC.