Does Firefox route WebRTC through a socks proxy? Does it leak your locally configured IP when WebRTC is initiated?
Even if the specific case of Firefox can be configured correctly and you have the source to verify that's it only sending traffic over the socks proxy, manually configuring every app to use a socks proxy is brittle and error pone, and for some apps just won't work.
Much more straightforward to just have a system wide VPN as the only available route for outbound traffic so that all apps use it transparently.
Even if the specific case of Firefox can be configured correctly and you have the source to verify that's it only sending traffic over the socks proxy, manually configuring every app to use a socks proxy is brittle and error pone, and for some apps just won't work.
Much more straightforward to just have a system wide VPN as the only available route for outbound traffic so that all apps use it transparently.