Hi Yes If you have your VPN (Desktop App) turned on and then toggle the Built-in VPN Firefox on: Your Firefox traffic will be encrypted by Firefox, sent to Mozilla's servers and then encrypted again by your private VPN and sent to its servers. Because your data is traveling through two different tunnels and being encrypted twice, your browsing speed will drop. Many websites will confused by the "hop" between two different VPN IPs and may block you, trigger endless Captchas, or simply fail to load. So, If your VPN is already on, keep the built-in VPN Firefox OFF. Your private VPN is already protecting your Firefox traffic. If your private VPN is OFF, the build-in VPN Firefox is a great "quick switch" for one more layer of privacy while browsing.