I use PureVPN which has a split tunneling mode. Waterfox is the only program to date I’ve found that doesn’t work properly with it. If I launch Firefox or Chrome through my VPN and check Whoer it shows the VPN’s IP address but Waterfox still shows my actual IP in addition to the vpn’s IP
The other way to disable WEBRTC is to go to about:config in your URL bar, search for media.peerconnection.enabled, and double-click it to set it to false.
Works fine with me using a vpn.
I’m using it now and it always shows my VPN IP.
You probably haven’t got WEBRTC disabled or something and are leaking your real address.
For a quick fix trying installing this addon: https://addons.mozilla.org/en-US/firefox/addon/privacy-settings/
Then click the shield icon and click on privacy (compatable) and security and then retry whoer website.
I’d personally would change this setting as it’s a bit deeper than just the privacy setting for the page. It’s more reliable than using an Addon.
If you have NoScript or uBlock Origins, they both have this option built in. I’d still use the about:config method first with the other addons as a secondary. There are some additional things that both NoScript and uBlock Origins do block by default when that setting ticked. Just extra measures 