Mozilla is building Thundermail as a new open source webmail project.
The service that handles onboarding and settings is called thunderbird-accounts. The JMAP webmail client is Stormbox. There are some related services, Send for large-file sending and Appointment for scheduling appts by email.
Thunderbird from iOS is being built from scratch as well.
https://github.com/thunderbird/stormbox
https://github.com/thunderbird/appointment
https://github.com/thunderbird/tbpro-add-on
https://github.com/thunderbird/thunderbird-ios