Firefox tweaks for the enlightened.

[Mario]

How to disable WebRTC in Firefox?

WebRTC is a communication protocol that relies on JavaScript that can leak your actual IP address from behind your VPN. While software like NoScript prevents this, it's probably a good idea to block this protocol directly as well.

WebRTC is a free, open project that provides browsers and mobile applications with Real-Time Communications (RTC) capabilities via simple APIs. The WebRTC components have been optimized to best serve this purpose.

The WebRTC initiative is a project supported by Google, Mozilla and Opera, amongst others. This page is maintained by the Google Chrome team.

In short: Set "media.peerconnection.enabled" to "false" in "about:config".

Explained:

Enter "about:config" in the firefox address bar and press enter.
Press the button "I'll be careful, I promise!"
Search for "media.peerconnection.enabled"
Double click the entry, the column "Value" should now be "false"
Done. Do the WebRTC leak test again.

If you want to make sure every single WebRTC related setting is really disabled change these settings:

media.peerconnection.turn.disable = true
media.peerconnection.use_document_iceservers = false
media.peerconnection.video.enabled = false
media.peerconnection.identity.timeout = 1

Now you can be 100% sure WebRTC is disabled.

WebRTC IP Leak VPN / Tor IP Test

February 2018 update: I have all settings as above, using Firefox 56.0.2, and the test fails. This may no longer work with "newer" browsers.

[Mario]

How to disable (opt out) the disgusting and hateful and spammy garbage that is "Heartbeat".

about:config > extensions.shield-recipe-client.enabled > false