if you downloaded from thunderbird.net, it is safe.

Thunderbird 115.14.0 esr is the most current of the older Tb 115 ESR channel and came out when Tb 128.1.0esr was released also on Aug 6th, Maybe your VPN (NordVPN) is not aware that Tb 115.14.0esr is a current update. Released on Aug 6th. https://www.thunderbird.net/thunderbird/115.14.0esr/releasenotes/ https://www.thunderbird.net/thunderbird/128.1.0esr/releasenotes/ And you can see Tn 128.1.0esr and Tb 115.14.0esr is the most current at https://www.mozilla.org/security/known-vulnerabilities/thunderbird/ and your VPN are only aware because Mozilla and Thunderbird are open about it.