Greetings

Open Firefox and go to:

  about:preferences#privacy  
  (You can type this in the address bar and press Enter)
  

Scroll down to the "Privacy & Security" section

Under "Permissions", click "Settings" next to "Camera" and "Microphone" to disable them:

 Uncheck the boxes for "Allow" or "Ask" for camera/microphone access.
 

This prevents WebRTC from using your camera/microphone, but WebRTC itself is not disabled (it can still use your IP address).

Close and reopen Firefox for changes to take effect

Best Regards

Why do you want to disable WebRTC? There is a "master switch" for this, but it may negatively affect other media-related features, so if you have a specific concern, it would be better to make a less drastic change.

Settings to prevent IP address leak on VPNs

See: https://www.reddit.com/r/firefox/comments/8hjh3h/google_voice_psa_if_you_have_been_recently_having/dylurz4/

Master Switch

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste media.peerconnection.enabled and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false