"Thundermail" will be part of a suite of Thunderbird Pro services, as the team behind the venerable Mozilla email client begins building a complete ecosystem.
From my understanding thunbird is somewhat separated from this. From the article linked by OP it says:
What’s crystal clear is that Thunderbird’s ever-increasing donation revenue (currently its sole source of income) is allowing for some explosive growth that’s long overdue. To add some context to this, Thunderbird received $2.8 million in donation revenue during 2021. Two years later, in 2023, it received $8.6 million in donations. I’m told that total financial contributions for 2024 were even higher, though the final amount hasn’t been officially released.
From my understanding thunbird is somewhat separated from this. From the article linked by OP it says: