Enterprise does not mean it is owned by a corporation. Also I think Mozilla is a non profit but I might be wrong there.

Enterprise is about who the target user is. If it is designed to run on a single user desktop or whether its supposed to integrate into central management and authentication structures. Enterprise software is most of the time sold as part of a service agreement with the developer offering updates and support for special customer use cases.

Thunderbird is simply a universal mail client that can be used by anyone. Privately and in a corporate setting