Edit: I’m assuming that the “it” you’re referring to is the “Disable WebRTC” extension, but with regard to whether disabling WebRTC is useful if you have an always-on VPN:
Here's an assortment of links with information about WebRTC
Accidently forgot to disable WebRTC in Firefox while on a VPN
Also, Mullvad has an article about WebRTC that lays out steps to disable WebRTC in Firefox without an extension (under the “Firefox (desktop) – advanced method” heading):