Not really, no, it is not. 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

Not really, no, it is not. 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.