If you’re using a VPN there is a good chance that your real IP address could be being leaked by your browser. Most browsers are affected but Firefox is the easiest to “fix”
To check, visit https://ipleak.net/ If your real IP address or that of your ISP are shown then you are not browsing anonymously.
Open Firefox and in the address bar type about:config
Search for media.peerconnection.enabled and change it to disabled
Now check https://ipleak.net/ and you should be safe to browse!