Long story short, you made the right choice. You can't purchase anything from a non-profit and you can't donate anything to a corporation.

If you purchase a product from them, it goes to Mozilla Corporation which makes all the products[0], if you donate money to Mozilla, it goes to the foundation.

[0] Minus Thunderbird, Thunderbird is developed by a separate foundation. Both the "Thunderbird foundation" (not actually called that) and Mozilla Corporation are 100% owned by the Mozilla Foundation.