Hi, it would seem that when using Private Browsing that Firefox is disabling the VPN Browsec as it needs the information that is being blocked by Private Browsing to perform/work.

This would appear then to be a issue that the Developer of Browsec needs to know about and to fix the issue, or not. Their Support site seems to have a login to your account : https://browsec.com/en/ Support email : support@browsec.com

Please let us know if this solved your issue or if need further assistance.

Some extensions need to use local storage and won't function in Private Browsing mode because in PB mode local storage and IndexedDB aren't available. The extension may still function in some way, but you can't save changes.