Do you mean, you never want to use WebRTC ever ever, or you want to adjust Firefox settings to make sure WebRTC can't leak your IP address while you are using a VPN, but keep the option to use WebRTC over VPN?

This older reply has info on WebRTC settings: https://support.mozilla.org/en-US/questions/1303685#answer-1348340

Lastest version not working this method

I hadn't tried using unlock origin..interesting. The older answer doesnt work as for some utterly unfathomable reason the developers disabled about:config! This forces privacy conscious users to use nightly or beta as their daily driver