Firefox has a feature which forces all DNS lookups to take place over predefined DNS servers over HTTPS (called DNS over HTTPS). With this feature enabled, Firefox uses the Cloudflare DNS servers by default.
If you want to avoid using Cloudflare's DNS servers and instead use the OVPN provided DNS servers while connected to OVPN, you will need to disable this feature in Firefox. You can do so by following these steps:
- Open Preferences in Firefox
- At the bottom under General, locate Network Settings
- Press Settings under Network Settings (located next to Configure how Firefox connects to the internet)
- Uncheck Enable DNS over HTTPS