Remember what happened to Thunderbird, though. Mozilla dropped it, and it got better. There are good reasons to think that Mozilla - the corporate entity - is cancer.
T'bird is owned and run by a for-profit company called MZLA.
That company is owned by the non-profit Mozilla Foundation but Mozilla does not own, run, operate, or control Thunderbird.
Comparison: Pret a Manger, the fancy sandwich chain, is part-owned by Mcdonald's. But you can't buy a Big Mac in Pret, and you can't buy a Pret fancy noodle salad in McDonald's.