

Disable WebRTC for Firefox – Defend Your Local IP(s) While Using a VPN - ChrisAntaki
https://addons.mozilla.org/en-US/firefox/addon/happy-bonobo-disable-webrtc/

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nacs
uBlock Origin (on Chrome and Firefox) can do this too:

In your uBlock options, there's a setting labelled "Prevent WebRTC from
leaking local IP address" (its unchecked by default since it can break some
WebRTC-powered sites).

