... New Moon 28 DOES NOT support WebRTC and the same stands true for (upstream) official Pale Moon 28/29; this has been a long-standing decision made by the Moonchild team, I think after a user poll; main considerations are security and performance toll on the browser... The team currently spends very little time on the WebRTC part of the (UXP) platform code, which is mainly still present (inherited initially from Mozilla ESR 52.6.0 platform) for the sake of Basilisk 52.9.x ...
In theory, you could build yourself a custom build of New Moon 28/UXP with the --enable-webrtc config flag, but if you are a social media sites fan (where WebRTC is mostly used), then opt to use for that purpose Serpent 55.0.0/Serpent 52.9.0, where WebRTC is built and enabled by default... Or switch to another browser (are you on XP?) with WebRTC support (probably all Chromium derivatives and, though I don't use it myself on my Vista laptop, MyPal 28.9.3) ...
Personally, I avoid ALL social sites like the plague (for security/privacy/performance issues, etc.), so I have currently no need for WebRTC and the like; pretty happy then with NM28 not supporting it... However, I am a democratic person by conviction and respect the right of other people wanting something I consider a nuisance...