Mozilla Thunderbird for Android is now available in beta, built upon the K-9 Mail app.
The beta includes core email features like account setup, email organization, and notifications, with feedback encouraged from users.
Thunderbird for Android will remain a separate app from K-9 Mail, requiring users to migrate if they previously used K-9.
I’d say customizability. Haven’t used k-9 since I found FairEmail so can’t really say anything about how it is now but it used to be pretty bare bones. At least compared to FairEmail.