The Mozilla Thunderbird email application is having a resurgence right now, with development ramping up and an expanded team behind the project. Mozilla has now confirmed how Thunderbird will be ported to Android.
Thunderbird is almost 20 years old, and duplicating all its features in an entirely new codebase on a new platform would likely take several years, at a minimum. That's why the Thunderbird team is taking a different approach: adopting K-9 Mail for Android. K-9 is also an open-source project (named after the robot dog from Doctor Who), and it's probably the most feature-packed email app on Android that doesn't require external servers or proprietary services.
Christian Ketterer (also known as 'cketti'), the maintainer of K-9, has already joined the Thunderbird staff. Mozilla said in its announcement, "Thunderbird will devote financial and development resources to improving K-9 Mail, adding new features, and user interface enhancements." The goal is for K-9 Mail to be rebranded as Thunderbird for Android, once enough progress has been made.
Mozilla's current roadmap for K-9 includes improved folder management, the same account auto-configuration as Thunderbird on desktop, message filters, and some level of synchronization between desktop and mobile using Firefox Sync. Thunderbird's non-email features (calendars, tasks, feeds, etc.) won't be in the mobile app, at least initially --- the team is "still debating how best to achieve that."
Since K-9 Mail will slowly become Thunderbird for Android over the next several months, you can technically get Thunderbird on Android right now by installing K-9. However, Mozilla is warning people that the interface "will probably change a few times" as development progresses.
Thunderbird also plans to offer an app for iPhone and iPad, but there's still no (public) plan for that yet. K-9 Mail is not available for iPhone and iPad, so Mozilla will either have to start from scratch for Apple's platforms, or acquire/fork another mobile app to rebrand.
Source: Thunderbird