WebRTC in Mozilla Firefox is partially supported since Firefox 22, and it's enabled by default.
To disable WebRTC in Firefox, go to about:config and toggle media.peerconnection.enabled to false.
Disable WebRTC in Chrome
WebRTC in Google Chorme is supported and enabled by default since Chorme version 23 (and browsers based on it, ex. Opera).
You CAN'T turn off WebRTC on the desktop version of Google Chrome.
You are only allowed to disable device enumeration. Go to chrome://flags/ and toggle Disable WebRTC device enumeration flag, that prevents any attempts to call media devices. All other WebRTC flags available only on Chrome for Android.
There is a Chrome Extension: WebRTC Block, that does nothing, just simply blocks WebRTC and related functions execution. Click here to get it.